Sento Kiryu

Sento Kiryu (桐生 戦兎) is a genius theoretical physicist who experiments with Fullbottles to further enhance his abilities as Kamen Rider Build (仮面ライダービルド) to fight against the evil Smash organisms. A man with no memory of his past, he was initially believed to be someone named Taro Satou (佐藤 太郎), but his true identity turned out to be someone else. Since the war for the Pandora Box broke out between the three countries, Sento accepted reality for what it was in order to end the war that his previous identity had started. Since the opening of Pandora Box and the resurfacing of his memories, he banded with his former enemies and fought against a common enemy who threatened to destroy Earth.

Early Life
Sento was originally Takumi Katsuragi, a research prodigy at the Touto Institute of Advanced Matter Physics, and nicknamed the "Devil's Scientist". One of the founders of Faust, he was responsible for the creation of Smash, the Transteam System, and the Rider System.

At some point, Katsuragi found that the former astronaut Soichi Isurugi and the ex-MMA fighter Ryuga Banjo are an evil alien entity known as Evolto and they came to destroy the Earth as they did with Mars, before they were able to do it, Katsuragi decided to hide the Evol-Driver and lure the two men into his apartment in order to kill them. Katsuragi confronted Evolto as Blood Stalk, after confronting him, he attemped to transform into Kamen Rider Build GorillaMond, however Stalk somehow knew that it was a trap and switched the Build Driver with its prototype, causing Katsuragi's Gorilla Fullbottle to revert back into an Empty Bottle, then Stalk ordered Katsuragi to tell him the location of the Evol-Driver which Katsuragi refused. As Stalk knocked out Katsuragi, a man named Taro Satou arrived at Katsuragi's home to apply for a drug-testing job, which was part of Stalk's plan, only to see Stalk and run in fear. Before Taro reached the door, Stalk killed him and switched both men's faces and their clothes. Now with Satou's appearance, Isurugi wiped Katsuragi's mind after Ryuga was arrested in Katsuragi's apartment. Taken into Faust's lair, the amnesiac Katsuragi watched in horror as he was injected with Nebula Gas with Blood Stalk and Night Rogue, a black suited person with bat features, oversaw the process.

When Katsuragi did not become a Smash after the injection, Isurugi realized that Katsuragi would be useful in gathering the rest of the Fullbottles. Isurugi smuggled Katsuragi out of Faust, making it appear that he found the amnesiac man outside of the café Nascita, Isurugi's home and front. Katsuragi was given the name Sento Kiryu and taken into Isurugi's care, hidden in the secret basement laboratory inside Nascita.

Becoming Build
At some point after being taken in by Soichi, he was provided the Build Driver and a set of Fullbottles that Soichi "stole" from Faust. During Sento's first fight against a Smash, he fought hand-to-hand but was beaten to the point where he had no choice but to use the Driver. Upon his first transformation, Sento defeated the Smash and headed back to Nascita, where Soichi and Misora were waiting for him. From that day on, Sento would continue being Kamen Rider Build in order to one day uncover the mysteries surrounding his past.

The Rabbit Tank Meets Dragon
After about a year of fighting as Build, Sento had gained some notoriety as the Kamen Rider. However, Sento's search was left at a standstill, as the Smash had no memories of their own experiments upon returning to human form. Meanwhile, Soichi, growing impatient with Sento's freeloading, directed him to a mid-level job at Touto Institute of Advanced Matter Physics.

Sento's first lead came in the form of Ryuga Banjo, an ex-fighter and wanted fugitive. Alerted by Ryuga's Smash energy readings, Sento followed and eventually apprehended him. Much to his shock, Ryuga had just escaped from the labs where Sento had been experimented on and had all his memories intact. The two were interrupted by Strong Smash, who attempted taking Ryuga away before Sento defeated it as Build. The Touto government quickly arrived and attempted to take Ryuga back into custody. Believing Ryuga's claim that he had been framed, Build helped Ryuga escape from the government, making Build a fugitive as well.

Burn Smash Incident and Meeting Night Rogue
Sento hides Ryuga in the secret lab, with Soichi scolding Sento for committing a felony. Sento explained that Ryuga was the 'best match' to his memories as they are both experimented by the same scientist group. On top of that, with exception of the location of the underground lab, Ryuga still had his memories intact. Sento deduced that Ryuga might lead him to the bat man and starts questioning him. Ryuga began the story with his birth, but Sento cut him off, stating not that far and scolded him, infuriating Ryuga and forcing him to shake Sento by the shoulders for insinuating him as an idiot. Sento told Ryuga to just start at the point of his arrest, and Ryuga told them what happened: he showed up at Katsuragi's apartment for a job, but he found him already dead at the same time the cops arrived. Isurugi objected jesterly, finding it strange for the local authorities to arrive just as Ryuga found the body, but Ryuga claims that what happened until a silver booth went off and opened a panel, unveiling the Gorilla Fullbottle. Sento runs toward it and gazed at the bottle before he commented the booth's work, which annoyed Misora as she came out and approached Ryuga, wondering who he was. As Sento theorized that the murder and Ryuga's jailbreak must have a connection until they were approached by Sawa Takigawa, a reporter. Both Sento and Isurugi were shocked as how Sawa found out about them, she showed Sento the Nascita matches set he dropped the night he saved her from the Needle Smash, and Isurugi realized he forgot to close the door behind after Sento brought Ryuga to the lab, but all four were completely unaware that Sawa was a spy for Faust and she knew the lab's entrance, grabbing a match set from a bowl to back up her cover. She then claimed to want to do a cover story on the Kamen Rider, but Sento declined until Isurugi agreed, much to the annoyance of Sento and Misora.

The next day, Sento, as Build, responded to a Smash alert, unaware it was a trap to lure him out by Faust. At the location, Build phones Isurugi to let him know there wasn't a Smash in sight until a squad of Touto Guardians appeared and open fire at Build. Build headed down near the edge of the docks and canceled his transformation, pretending to be an average joe until the Guardians left. At the Touto Institute, Sento ran into Gentoku and a few researchers watching the news as they learned that Touto has declared the Kamen Rider as a fugitive until he received a phone call from Misora, who informed him Ryuga escaped with her phone, both unaware that Ryuga learned that his fiancé, Kasumi Ogura, has been abducted by Faust as well. As Build, Sento tracked Misora's phone to an abandoned warehouse where Ryuga is being pursued by Touto Guardians. Utilizing a Vortex Break with his Drill Crusher gun mode and the Harinezumi bottle, Build asked Ryuga why he left, to which Ryuga stated he has a girlfriend he intended to marry. After defeating the Guardians in the warehouse, Build and Ryuga ran into more Guardians that merged into their Union State and opened fire, forcing Build and Ryuga back into the warehouse. Inside, Build turns into the newly acquired Gorillamond Best Match and proceeded outside, formulating a plan to take down the Guardian Union State, which included knocking Ryuga's motorcycle inside an abandoned building, allowing the Guardian Union State to pursue. As the machine made it to the right spot as its gunfire destabilized the building, Build used the Muscle Glove to destroy the machine.

As they regrouped, Sento asked Ryuga about his fiance, who revealed she was also the one whom sent him to Katsuragi's place for the job. As Sento asked why he didn't say anything, Ryuga punched him in the gut and put him in a chokehold, rendering him unconsciousness to prevent his involvement. However, Sento came to early and managed to catch up and was too late seeing that Kasumi was already transformed into the Burn Smash. As Night Rogue appeared, Sento turned into Build HarinezumiTank Trial Form and fought Rogue to demand answers for his experimentation, but Night Rogue responded that he can't remember every guinea pig Faust had, angering Build. Rouge send Build back with his Steam Blade, telling him he can't beat him with a random bottle. As Build rose, he noticed the Burn Smash preparing to attack Ryuga and pushed him out of the way as the Smash blasted them with a fireball. As Build prepared to fight, Ryuga pleaded to not fight Kasumi, but Build assures him she can be saved once the Smash element has been extracted from her body. However, Rogue intervened and told them if they do that she dies as her Hazard Level is 1, meaning her body's weak and injecting it with the element is fatal as he takes his leave. Build attempts to engage the Smash, but Ryuga stops him until Build spotted the Smash attacking itself. Build stated that when a person becomes a Smash their soul disappears, but Kasumi's soul is still in the Burn Smash as it struggles to not attack Ryuga. Realizing that Kasumi cannot be saved, Build agrees to separate the Smash from Kasumi's soul long enough for Ryuga to say goodbye. first, Build turned into his RabbitSoujiki to absorb all the fire with the Long Range Cleaner, then released the fire with the Trash Converter to create a fire cyclone.

Turning into the GorillaMond Best Match, Build turned the fire into small diamonds and performed a Vortex Finish to place the Burn Smash in a diamond cyclone that separated it from Kasumi's soul. However, he used the Muscle Glove to keep the Smash at bay as he told Ryuga he couldn't do anymore. After Kasumi passed, Build tossed the Smash's body aside to finish the attack and absorbed the element in an empty bottle. As Sento told Ryuga to leave with him, Ryuga broke down with grief and said it's fine if he's incarcerated once more. Angered, Sento reminded him if the authority finds him, he will still be branded as a criminal and Kasumi's life will have been sacrificed for nothing. They headed back to their motorcycles and left the scene. Back at the lab, Sento fell asleep as Misora purifies the Burn Smash bottle, unaware that Ryuga thanked him for letting him see Kasumi one last time until the booth was finished. Upon seeing the newly purified Dragon bottle, Sento tossed it to Ryuga, stating he should hold onto it and to thank Sento after he proved his innocence. Sawa appeared, informing Sento and Ryuga of the man who drugged Ryuga: Masahiro Nabeshima, the same man whom Kasumi mentioned before her death. Now with the connection between the murder and Ryuga's jailbreak, Sento planned to find Nabeshima.

Mirage Smash and Blood Stalk
The next day, Sento explained to Ryuga that Nabeshima was responsible for using Kasumi to make Ryuga Katsuragi's murderer, but also drugging him and placed him in a Smash laboratory. Misora pointed out that finding him would be impossible as his resume was a fake, much to Sawa's sorrow. Misora offered to help find information on Nabeshima, but Ryuga appeared skeptical until Isurugi assured him not to underestimate Misora, who was revealed to be a famous net idol called Miitan. In a matter of minutes, they've received information, but not the right kind: a Smash has appeared. Pausing the search, Sento sets out to intercept the Smash, but Ryuga questioned him whether being a Kamen Rider or finding Nabeshima, the person connected to the Smash and framing Ryuga, was important. Sento states Build is before leaving. Upon taking down the Flying Smash, Sento used the extracted essence to create the Taka Fullbottle. Meanwhile, having obtained Nabeshima's contact information, Ryuga began heading west to Seito with Sawa. Saving Ryuga from a group of Guardians, Sento advised him not to mistake justice for revenge before being attacked by the Mirage Smash. Build neutralized it only to find that the Smash was Nabeshima. Before he had time to react, he was poisoned by Blood Stalk and fell unconscious.

Sento awoke in Nascita's basement, having allegedly been saved by Isurugi.

to be completed

Uncovering Faust
Once again at a dead end, Sento decided to examine Katsuragi's research notes, finding a message to Katsuragi's mother Kyoka in the final one. Escaping another attack from Rogue and Stalk (mainly because Stalk's interference), Sento and Ryuga snuck to Hokuto to speak with Kyoka.

to be completed

On November 30th, Sento and Ryuga were neutralizing a Strong Smash Hazard when they rescued a mysterious young man named Sougo Tokiwa, who claimed to hail from one year into the future. Taking him with them to back nascita, Sento examined the Ridewatch on Sougo's person, confirming through its technology that it did indeed hail from a future time period as Sougo had claimed. Soon after, Tsukuyomi entered nascita, using her Faiz Phone X to stun all three men before returning Sougo to his own time.

Blood Stalk Revealed
"Just now...the Soichi Isurugi I knew died."

- Sento severs his ties with Soichi after learning his true nature.

On the night that Ryuga became Cross-Z officially, Sento was called by Nabeshima, who had regained his memories. Nabeshima revealed he had framed Ryuga on Stalk's orders, and that Isurugi was Stalk. Shaken by this discovery, Sento secluded himself in the basement until Isurugi returned. While reluctant to believe Isurugi was Stalk, Sento created counterfeit Fullbottles to be safe. When confronted with his odd behavior, Sento told them what Nabeshima told him, with the exception of Stalk's true identity.

To test Nabeshima's claim, Sento had Misora purify the Octopus Fullbottle in secret. On a tip from Gentoku, Sento and Ryuga raided a building for Nanba Heavy Industries, where the Pandora Box was being held. Confronted by Stalk, Build and Cross-Z were unable to match Stalk until Sento used the newest Best Match: OctopusLight. Unfamiliar with a Best Match he had never seen, Stalk was easily defeated, confirming Sento's suspicions. The Pandora Box was brought to Nascita, where the group celebrated with dinner. In the middle of the night, Isurugi snatched the Pandora Box, its panel, and the Fullbottle decoys. Before he could leave, Isurugi was confronted by Sento, who had been waiting for him upstairs. After initially running away, Isurugi admitted his ruse and transformed into Stalk to fight.

During the night battle, an essence of Paradox fell into one of Sento's empty bottles and decides to infuse it with the Best Match from a pair of 10 pairs existence after researching on how to use it, resulting on RabbitTank which would be infused after RocketPanda, GorillaMond, and LionCleaner proved to be failure with Ryuga to be the one who tested them all. Sento later challenged Isurugi again, almost to be defeated because of being reluctant until Ryuga came to his aid after distracting Misora by a sudden dating distraction plan from Sento. After Ryuga had lost instead, Isurugi told Sento and Ryuga that the Kamen Rider project is nothing more than a weaponry project and not for becoming heroes of justice. Isurugi also told Sento that he, Misora, and Ryuga were only pawns for the fruition of his plans. Angered, Sento used RabbitTank Sparkling, his completed upgrade, to defeat Stalk and take back the Pandora Box. Isurugi told Sento to search on Takumi's hidden project and claimed that Sento definitely able to finish it and telling that Night Rogue is Gentoku before bidding farewell.

Learning his Past
Sento and Ryuga promptly confronted Gentoku, attempting to get a confession. While Gentoku fought them both off, he advised Sento to look up "Sclash" in Project Build. This hint led Sento to the Sclash Project, the evolution of Project Build. In an excited frenzy, Sento finished the necessary computations within a night. Unbeknownst to Sento, this had been set up by Isurugi as he sneaked into his cafe and copied the data before leaving.

When Gentoku faced and lost against Rabbit Tank Sparkling, Gentoku revealed that Sento was actually Takumi and explained how everything had been set up by Isurugi from behind the scenes.

Overcoming his Guilt
"This is the worst. Having you criticize me...I remember now. I...am a narcissistic-ego driven...hero of justice!"

- Encouraged by Ryuga, Sento regains his confidence as a Kamen Rider.

Sento became overridden with guilt due to Gentoku's words, working silently on the Sclash Driver. Ryuga rushes towards in front of him, knocking over the unfinished driver and addresses Sento that he's responsible for his girlfriend's death and all the people who became Smash. Sento demanded what Ryuga wanted, and he replied to fight. As Ryuga gains the upper hand, Sento recalls past events that enforce his true identity as Takumi, until Ryuga notices he wasn't fighting seriously. Obliging to Ryuga's demand to fight for real, Sento changed into RabbitTank Sparkling and finished Ryuga with a Rider Kick. After the battle, Sento shows remorse for his crimes as Takumi and leaves a conflicted Ryuga with Misora watching on.

Hokuto Invasion
One day, Sento received a new pair of Fullbottles: Kuma and Televi, which had been relinquished to Ryuga by Grease as his penance after the Hokuto Three Crows interrupted their battle. Soon after, Sento faced Massugu Ubukata, a man from Hokuto who, in a bid to reclaim the Bottles, held Misora hostage while using a Smash Bottle to become an Ice Smash. Familiar with this kind of Smash, Sento assumed NinninComic Form to fight him as before only to find that its freezing power was greater than the first, leading him to switch to the new Best Match, KumaTelevi. True to its name, the Televi Halfbody provided a television broadcaster whom advised Sento to weaken the ice by introducing an impurity, which he did by infusing it with honey from the Kuma Halfbody before enlarging himself and defeating the Smash with a slash from the Kuma paw. At this point, Build was confronted by Grease, and learnt from Televi-san that Massugu was indebted to Kazumi after he helped save the life of his grandmother. Realizing this, Sento chose to return the Kuma and Televi Fullbottles to Kazumi while suggesting to Massugu that he would do better to go back to Hokuto and continue supporting his grandmother.

"I will stop you. Even if I lose myself!"

- Sento's before transforming to RabbitTank Hazard Form for the first time

Forced to stop the rampaging Cross-Z Charge, Sento used the Hazard Trigger to transform into RabbitTank Hazard. Despite his efforts, Sento was unable to stop Ryuga before the Hazard Trigger's effects took hold, sending him into a mindless rage. Sento easily took out Cross-Z Charge, Akaba, and Kiba before fatally wounding Aoba. Knocked out of his transformation by Grease (who arrived seconds too late), Sento regained consciousness to watch Aoba's death in fear.

The murder left Sento devastated and traumatized: he surrendered all of his Bottles to Touto's government and entered a near-catatonic state.

The next day, Sento was visited by Taizan to inform him about a proxy one on one battle to end the war between Touto and Hokuto for good. Sento had initially refused to fight, however meeting with Soichi changed his mind, having received all the Fullbottles in the possession of both Hokuto and Seito from him.

Sento began work to figure out all the Best Matches in his lab, much to Misora's sadness as she began to worry for Sento. Utsumi then appeared at the lab and reveals he was asked by Soichi to help Sento.

However when Seito attacked, a Bikaiser appeared and began to attack Sento back, but he defeats it with several Best Matches until he realized that it wasn’t Kaisei Mogami, but a random victim sent by Seito. Sento returned home where Misora treated his wounds. After a all-nighter, Sento had created a kill switch for the Hazard Trigger in case he goes out of control. After some thought, he realized the only person he could trust the kill switch with was Misora. He presented it to her the night before the duel, much to her reluctance to accept it.

On the day of the duel, Sento went up against Kamen Rider Grease, who Sento was able to overwhelm for a while with several Best Matches. However Grease was suddenly motivated by an increase of power and began overpowering Build. Sento's is forced to play the RabbitTank Sparkling card from his hand, only to still be overwhelmed by Grease's "passion." Build is hesitant from using the Hazard Trigger for a short moment, before activating it and entering RabbitTank Hazard. However he goes berserk and defeats Grease with no mercy, to the point where he was prepared to kill Kazumi even after his transformation was disengaged. However with Sawa's warning, Ryuga jumps into the ring in order to stop the berserk Build and eventually cancel his transformation. Misora later returns the unused kill switch to Sento.

Victory over Hokuto
However as Kazumi is called back to his homeland to rescue its citizens from the Washio brothers, Sento and Ryuga were given the offer to assist Kazumi, but they decline due to allegiance issues, but they promised to meet them at the Sky Wall once they were done. Kazumi manages to return to Sento and Ryuga with Akaba, however they were followed by the Washio Brothers and engaged in battle with them. Build and Cross-Z are able to push the Washio brothers back and force them to retreat, but not before the sudden appearance of Gentoku Himuro, who had allied with Seito as Kamen Rider Rogue. Rogue easily defeats the combined efforts of Build and Cross-Z without breaking a sweat.

War with Seito
Sento regains his composure after his utter defeat from Rogue, and sent his regards to the deceased citizens of Hokuto, including the Hokuto Three Crows. However Blood Stalk confronts him saying that his own views were not nearly enough to win, much less end this war. Sento is provoked into using the Hazard Trigger against Blood Stalk, and his rampage is luckily stopped by Grease who interfered. After Stalk retreats, Kazumi and Akaba are invited to live at nascita for the time being, and Sento begins questioning his own motives, but reaffirms those beliefs after a pep talk from Misora.

Later the day, the duo receive a call from Prime Minister Himuro, saying their office building was under attack by Seito. Build, Cross-Z, Grease, and Akaba rush to help and encounter Washio brothers in combat. Build personally heads up to the Prime Minister's office in an attempt to protect Pandora's Box, only to encounter Gentoku there, about to take it. He transforms and decides to take the battle with Build somewhere else. They arrive on the outskirts of the Sky Wall, where both Grease and Cross-Z arrive shortly thereafter. To combat Rogue's abilities, Build access Hazard Form but is completely outmatched even in his berserk state. Just as Rogue is about to finish a downed Sento, Misora suddenly appears and reveals the entity inside of her, Vernage, who uses her powers to force the Rogue and the Washio brothers over the Sky Wall back to their own territory. Just before Vernage disappears, she murmurs a name Evolto, and Misora falls unconscious. After bringing her back to nascita, Sento arranges a meeting with Soichi Isurugi to ask him about Evolto, but Soichi claims to not know anything and decides to stay away from Misora, after hearing of her newfound powers. He also drops information that there was a Seito spy within Touto, and reveals that the spy is a member of the Namba Children, children who were adopted by Namba himself and trained both science and espionage while swearing complete loyalty to him. Sento returns to nascita with this information, just when Misora regains consciousness. She has no memory of the attack and hopes to discover what was wrong. Sento helps by pulling the A.I. information from Misora's bangle and her mind, and they discover footage of Pandora's Tower destroying the Martian civilization, presumably from the viewpoint of Vernage's. Misora freaks out, but Sento calms her down and reassures that he will protect Touto and her, and find a way to end the war once and for all.

Ryuga and Sento devise a plan to weed out Seito's spy. By feeding a different location of Pandora's Box to several people, whatever location that the Seito forces would show up, the person who was given that location was the traitor. Build and Cross-Z confront the Washio brothers at the engineering factory, causing them to retreat once they realized the Box wasn't there. Sento and Ryuga return to Prime Minister Himuro's office, and accuse Masuzawa of being the traitor, as he was the one with the location that led to the engineering factory. With his cover blown, he commits suicide. However, the team realize that the true location of the Box was discovered by Gentoku Himuro, who appeared and attacked Akaba, eventually killing him. Grease, Build, and Cross-Z join together to fight Rogue, with Build going Hazard. Build's time of berserk allows Rogue to escape with the Box, and Build turns on Misora. However, Misora's bangle activates, allowing Build to break out of his rampage and regain consciousness, and deactivate his transformation.

War for Pandora's Box
No one should have known the true location of Pandora's Box except the team themselves, and yet Rogue managed to discover the location and escape with the box. With this information in mind, they attempt to uncover the traitor within their midst. They then meet with Prime Minister Himuro, who informs them about the 3-on-3 Proxy Battle with Seito. If Touto loses, then they must surrender their territory and ALL of the Fullbottles in their possession. If they win however, then Seito will cease any further invasion of Touto and restore Hokuto back to its former glory.

Sento begins work on a new upgrade for Build to use during the proxy battle. He stated that Hazard Form's biggest flaw is its increased stimuli overflow, thus causing Build to go berserk. Sento decides to make a new power-up that'll allow him to surpress the overflow stimuli all while keeping the power that Hazard Form can give, and then some. His theory comes from the power that Misora's bangle exhibited, and that it was able to restore Sento's consciousness. After the completion of a white Rabbit Fullbottle, with the help of a reluctant Misora, Sento tries it out on the Build Driver alongside the original Rabbit bottle and the Hazard Trigger, but it results in a explosion, and the test bottle is destroyed. Sawa arrives shortly after and says Kazumi is at Sky Road, so Ryuga goes to find him while Sento and Sawa work on the development of the power-up.

The two compliment each other for their technical skill while talking about Sawa's past. He questions Sawa about her father's past, telling her there were no recorded victims with the name "Takigawa", Sawa replies saying Takigawa was her mother's name. Sento had finally completed the FullFull RabbitTank Bottle and it's accompanying gear, and was about to head out, but he stopped to hear Sawa's story. She revealed to him that her father dying in the gas explosion was a lie, and she herself was one of the Nanba Children. Everything from her being turned into a Smash to being kidnapped by Touto was all part of her plan to gain Sento's sympathy and trust, and if her position was ever compromised, she'd have to commit suicide. However her feelings changed after being with Sento and his friends for so long, it gave her a "family" that she never had. She also revealed that Namba had Nabeshima's family hostage, and wanted Build's upgrade schematics in exchange for their freedom. The two formulate a plan. Sento gives Sawa just RabbitRabbit's data to give to Seito, while keeping TankTank a secret. Sento hurries to the venue of the proxy duel, and Ryuga teases him that Sento may not have to use it. Sento watches as Cross-Z falls to Remocon Bros' deceit. It was then time for Sento to fight against Gentoku Himuro in the final round. To fight Rogue, Sento begins with RabbitTank Sparkling in an attempt to break the even match, however his own motives are questioned by Rogue in an attempt to hypo-criticize him. Sento responded that he fully understood how much of the war was caused by Takumi Katsuragi because of his invention of the Rider System, and felt it was in his responsibility as the instigator of the war to end it. Build then reveals the new Bottle and transforms into RabbitRabbit, which is basically Hazard Form with a new layer of armor. Build in his new form is able to "defeat" Rogue by blasting him out of the venue, however that was not a condition for loss (or win), as Rogue has not given up nor had his transformation disengaged. After receiving RabbitRabbit Form's data, Gentoku gained the upper hand and continued to ramble on about his ambition and how it was the only thing he felt he could rely on, and quickly rejected Sento's idea of "love and peace". In the midst of battle, Sawa receives a call from Kazumi, saying the Nabeshima family were now safe. Giving Sento the signal, Build immediately transforms into TankTank Form and outwits Rogue, reaffirming that even though "love and peace" may not be the ideal form of belief in the reality they were currently in, it at least gave him the strength to fight for a new world. With Gentoku fighting Build blind from TankTank Form, Build is able to defeat Rogue by transformation disengagement, and secure victory for Touto. Prime Minister Himuro appears to congratulate Sento and attempt to bring Gentoku back to Touto, he however bitterly walks off, leaving Sento in worry. The team regroups after the match. Misora slaps but hugs Sawa. Sento thanks Kazumi for helping, but he takes the honor instead.

Seito's Betrayal
With Pandora's Box in Seito's hands, Blood Stalk activates the first full panel of Fullbottles on the Box and raises the first Level of Pandora's Box. With Seito now serious about an all out war, Build, Cross-Z, and Grease attempt to make their stand against the new Hard Guardians. News break out that Touto was the loser of the battle, and that Taizan Himuro was the one who was unwilling to stand down. In this new situation, the team attempt to figure out their next move. Kazumi wants to go back to Hokuto, but Sento said it was useless now that they were in a warzone. The team head out to battle the Washio brothers the very next day with newfound resolve, however in the midst of battle, Misora is held hostage and is ransomed for the team's bottles. However at that moment, Vernage reawakens and reveals her title as the Queen of Mars. Vernage forces Soichi and the Washio brothers to retreat, and the team treat Queen Vernage to a cup of coffee at nascita. Vernage reveals that her body was long gone, and her soul will not last forever in Misora's bangle. She also reveals that Evolto will eventually use the Pandora's Box to destroy Earth, just like he did with Mars. She then questions Ryuga's identity, saying he will be their final hope, before falling unconsciousness. The team head to the lab to discuss what Vernage said. Ryuga claims his childhood memories were vague. He picks up the rock that contained the essence of his former Sclashjelly and it begins to catch on fire. He drops it and it transforms into a new Fullbottle. Realizing something, Sento asks Sawa to look into Ryuga' background and decides to create a power-up for Ryuga using the newly created Fullbottle. After Sento finishes, both Misora and him walk out to discuss Vernage and ways to end the war.

Pandora's Tower
The next day, Sento's sudden resignation from Touto's military forces had charged him with treason. This, however, is all part of Sento's plan to enter Seito and retrieve Pandora's Box without triggering any allegiance issues. Build rides his bike to the Tower while being ambushed by Hard Guardians, but not without backup from Cross-Z and Grease. The three make their way into the Tower, but were met with the Washio Brothers and some Hard Guardians. During the fight, Build gives Cross-Z his newest weapon, the Cross-Z Magma Knuckle. Blood Stalk appears and manipulates the ground, due to his influence over the Pandora's Box. Later, Gentoku as Rogue would join the battle as well, with Cross-Z attacking him first but is defeated. Build heads to fight Rogue, but during the midst of battle, Rogue reveals that due to a chip implanted in him, he is forced to obey Seito's commands, and reveals that Masakuni Mido is in fact dead, and that Juzaburo Namba has impersonated him. He asks Sento to take back the Pandora Box to prevent Juzaburo Namba from taking over the entire world. Rogue intentionally lets Build defeats him, and Gentoku reverts out of Rider form. Considering Gentoku's words, Sento, Ryuga, and Kazumi had to the center of the Tower and confront Stalk who has the power of the Box under his control. Stalk summons two Clone Smashes to fight Build and Grease, while Cross-Z engages in combat with Stalk. Ryuga is defeated, and Sento rushes in to protect him from a blast that would have killed Ryuga, and ends up going down as well. As Sento is helplessly in Stalk's hands, Ryuga awakens his true potential and transforms into Cross-Z Magna and attacks Stalk, while Sento watches. Ryuga is successfully able to defeat the Clone Smash, drive Stalk away, secure the Box, and get the team out of there.

Sento brings the Box to Prime Minister Himuro and asks him to use the Box to negotiate and end the war. Sento and the Riders return to nascita, where Sawa then reveals Ryuga's extraterrestrial origins to everyone and that the man responsible behind it was Takumi's father, Shinobu Katsuragi. At first Ryuga doesn't believe it at first, but Sento reminds him that Cross-Z was somehow about to calm Pandora's Box using the same power as Stalk. Sento also cases on the fact that Ryuga's alien power was reset when he was born, and his alien origins would explain Ryuga's abnormal increases of Hazard Level. Sawa is then confused on why information like this was not "top secret", as her retrieval of the information was rather easy.

Sento is then called to Area D4, where he meets Grease to fight against the Washio brothers. The fight went well, with the Washio Brothers in defeat, however they were decoy to lure Build and Grease away from the Touto government offices, leaving it defenseless for Stalk to plunder. Sento hurries to the government building and finds Ryuga there already. He re-encourages Ryuga to stand up, and watches him fight Blood Stalk as Cross-Z Manga. He manages to drive Stalk away, but the two then realize that by fighting Stalk, they allowed Rogue to get into Prime Minister Himuro's office. By the time Sento and Ryuga arrive, Rogue had already left with his father.

Evolto's Plan
The team returns to nascita to discuss further options. Sento receives a call from Soichi, who demands the Evol-Driver that Takumi Katsuragi hid, in exchange for PM Himuro's life. Looking for a clue to where the Driver may be, Sento finds the file his father wrote and pulls it up. The file reveals that Evolto has taken over Soichi Isurugi and that he is the only one able to use the Evol-Driver. Sento recognizes the Driver's gear work and is able to find the Driver disassembled in the Fullbottle purifier. Sento then receives a call from Gentoku, who wishes to have Sento disable the chip inside of him so that he can help him get his father back. The next day, Sento and Ryuga arrive at the trade, with Misora and Sawa standby outside. Sento wants Stalk to hand Himuro over first, but Stalk insists Sento is in no position to make demands considering the situation. Then, Rogue, now with his chip disabled, appears and runs out with his father, while Build and Cross-Z stay behind to fight the Clone Smash. After defeating them, they go outside only to find that Evolto has trasnformed into Kamen Rider Evol, and Taizan Himuro dead. Realizing the imminent threat, Build, Cross-Z, and Grease enter battle with Evol. Sento however is defeated and poisoned by Evol, who said Sento has only three days left to live and the only antidote is with him. Sento is taken to the ICU and follows through with the plan for Ryuga to defeat Evol and cure Sento. Ryuga is able to defeat Evol, thus curing Sento. However Evolto takes this opportunity to possess Ryuga and access Evol's Dragon-based Phase 2 form.

In the midst of their despair, Evolto takes this chance to enter the central building and steal Pandora's Box, only for Sento to give chase in an attempt to stop him from doing so. Sento transforms into Build and battles Evol while he reveals his entire plan from the very beginning, including how Ryuga came to be and Evol's plans for him. In the end, Sento is utterly defeated and brought back to nascita to rest. After he woke up, he revealed that during his time poisoned, he had a dream that Ryuga was calling for help, and vowed to honor that. Realizing that Gentoku has the final Fullbottle needed for Evolto to open the box, he calls Gentoku to nascita to cooperate in protecting the last bottle. However Evolto shows up and teleports everyone to the core of Pandora's Tower. Evolto activates another completed panel and raises the second level of Pandora's Tower. In that moment, Build, Grease, and Rogue enter battle with Evol. Rogue attempts to distract Evol in order for Build to grab a panel and retrieve the bottles, however the plan fails and results in both Rogue and Grease getting flung out of the Tower.

Sento resorts to using the bottles for regular Hazard Forms in concession to battle Evolto, while being criticized of reverting back to his old tactics. After entering KeyDragon Hazard, Evol decides to enter Evol Dragon Phase 2. At that moment, Sento begins feel the stimuli taking over, but he suppresses with TankTank form. Evol begins to tell Build that science is the key to humanity's destruction, but Build retorts that people aren't so simple that they would so easily allow destruction to happen as they have the power to prevent it in the first place. Build is able to put Evol on the ground, however Evol impersonates Ryuga's voice, allowing Evol to attack Build back and defeat him. Sento helplessly watches as Evol inserts the third and fourth panels onto the Box, allowing the Tower to absorb the Sky Wall debris. Sento cries out as Evol activates the fifth panel, allowing for the Tower to complete itself. Evolto uses the power of all the Fullbottles to create an incomplete version of the Evol Trigger, which he activates to no avail. Then, Vernage appears and teleports her, Sento, and Pandora's Box back to nascita.

Back at the base, Vernage reveals how she began her stand against Evolto at her planet's destruction and says Evolto cannot be allowed to reach his true and final form. To help, she uses her power to restore the Fullbottles' essence, which prompts Sento to somehow figure out that the Panels can be used to help engineer Build's final power-up. However using it is fatal, which causes Misora to berate Sento about not taking care of his own life. Sento promises Misora that he'll come back.

Sento and Grease are called out to an attack at Touto by Seito, where Evolto leads several Hard Guardians to attack the public. Build as RabbitRabbit attacks Evolto with a concession of Fullbottle attacks where he is able to overwhelm Evol, but he changes to his more gracile Evol Dragon, and Sento responds with TankTank. The two exchange conversation that Evol thinks humanity is at fault for this war, but Sento believes the humans have the power to prevent it. At the brink of defeat, Build brings out the Fullbottle-shaped Panel power-up, and affirms that it's for the best, to protect his friends and end this war. He sets the power-up onto his Driver and begins cranking the lever. Evol approaches him but is knocked back after revealing that Build is now at Hazard Level 6. Evol is pleased as he reveals that the plan was going well. Evol strikes his hand inside Sento and releases Ryuga. At that moment, Build began to crank the lever once more, continuously raising his Hazard Level. Evol knows that if Build continues to do this, he will die, but Sento was prepared to take Evolto with him. The two explode, leaving only a white-haired Sento. Ryuga wakes up and tries to wake Sento up, only to reveal that Evolto has taken him over. With Sento, Evolto creates the Rabbit Evolbottle and transforms into Evol Rabbit, Phase 3.

Possession by Evolto
With Sento now in Evolto's possession, he attempted to activate the Evol Trigger, only to no avail, saying that the humans aren't strong enough to handle the power of Pandora Box. Ryuga attempts to transform into Cross-Z Magma, but Evolto tells him that because Ryuga left Evolto, he no longer has any of his essence left, and therefore cannot transform. Grease decides to try, but is utterly defeated. As Evolto attempts to leave, Ryuga follows but is knocked down before he could continue. Evolto returns to nascita, only to find the entire shop "gone" and immediately figured it was Vernage's doing.

After Evol gains his complete form, Black Hole Form, Sento is jettisoned from Evolto, only to be rescued by Ryuga. However, the result is that his memories of Takumi Katsuragi resurfaces, while his memories of being Sento are now gone.

Memory Resurfacing
Resurfacing with Takumi's memories, he is surprised that Evolto has reached his final form and watches the other Riders fight Evolto again. Shortly however, Gentoku realized that they were outpowered and forced the entire team to retreat back to nascita.

Back at nacita, Takumi is overwhelmed by his new look and continuously wonders how this was even scientifically possible. He began to scribble over his marker board while overhearing the others discussing his nickname "The Devil's Scientist", saying "devil" was uncalled for. He recognized Misora Isurugi as the girl whom he had purifying Fullbottles for while he was with Faust, all in the name of science. Takumi began to wonder why Ryuga Banjo was helping him, despite him having Evolto's DNA, and believes that even if the DNA was gone, Evolto could still awaken inside.

Takumi is called out to talk by Gentoku Himuro, and the two exchange words on how Gentoku misused the Rider System for war. Gentoku apologized and said the Pandora Box altered his mindset, but Takumi didn't want to hear excuses since it was undeniable that it was them both that were responsible for the war. Takumi knew that even though Gentoku wanted to use the Kamen Riders for war, he convinced himself it was solely to stop Evolto's upcoming attack, and that the sacrifices made were necessary. In the end, Takumi walks away, saying he never should have trusted Gentoku.

Later, the team enter Evolto's space of creation to help Utsumi on his request to stop Evolto. However, Takumi is unable to keep up with Build RabbitRabbit form as it was too high-performance for him. Evolto comments that Takumi will never be able to use the latest version of Build, but Takumi proclaims that he can and he will, because he built Build for the sole purpose of beating Evolto. Takumi attempts to use the Pandora Box panel, but is froze still by the electrical output of it. Evolto takes this chance to punch him back and force him out of his transformation. Evolto takes the later chance to kill both Rai Washio and Fu Washio, thus Takumi becomes angry to see lives being lost in front in him. Evolto hands Utsumi a Evol Driver and he transforms into Kamen Rider MadRogue, where he fights Gentoku. Gentoku is defeated and Grease arrives in the nick of time to help the team escape.

Back at nascita, the team pounders on what to do now that Evolto has voiced his intent to take over the country for himself, and they want to investigate why Evolto now wants to rule a planet rather than destroy it. Takumi wishes to complete the new power-up item to defeat Evolto, along with Ryuga, who is beginning to have episodes of Evolto's memories surfacing in him. Later that night, Takumi completes the Genius Bottle and is confident he can defeat Evolto and Ryuga. Misora berates him for not believing in his friends, but Takumi said friends can only lead to distrust and betrayal. The next day, Takumi hears Gentoku regretting his actions in the past and wants to make it up and asks the team to believe in him.

Takumi heads out to "help" Cross-Z and Grease with RabbitRabbit Form, now modified to fit his specs, and easily defeats the Clone Smash. He fights MadRogue to an equal standstill, but is knocked back when Utsumi questions his motives for science. Takumi reveals the Genius Bottle and tries to use it, but it wouldn't activate. Theoretically, he should be able to use it, but Takumi didn't know why he couldn't, as MadRogue attacks him and forces him out of his transformation. Ryuga arrives to fight MadRogue and tells Takumi that Sento fought for justice and was the epitome of a superhero, traits that Takumi didn't have. Watching MadRogue force Ryuga out of Rider form, Takumi experiences a flow of memories coming into his mind. Takumi then appears in front of Sento, and asks him if he believed in the power of trust and friendship. Sento responds with a smile, and Takumi asks Sento to show him the future he can build. Takumi and Sento join hands, and the two assimilate with each other into Sento.

Two Minds, Now United as One
With both the memories of Takumi and himself, Sento appears to tease Ryuga for losing, which the latter responds with an equally as witty remark. Declaring the return of the egotistic superhero, he reveals the Genius Bottle and successfully activates it into his Build Driver, thus giving birth to Kamen Rider Build's final form, Genius Form. Immediately, Build begins to overwhelm MadRogue with intense speed and power, even crushing the Nebulasteam Gun to nothingness and bringing the battle to a obvious victory. Evolto appears and is impressed by Build's new power. He attacks and takes the Lost Bottles in Kazumi's possession, saying he wants to take over the world with them, along with crushing the hopes of our heroes.

The team returns to the cafe, but both Sawa and Misora are doubtful that Sento has really returned, but he assured them he has. Seeing that Kazumi is depressed following Evolto stealing the Lost Bottles from him, Sento begins to wonder why Evolto is still collecting Fullbottles despite having already reached Black Hole form. Sento asked where "Gen-san" was, referring to Gentoku using honorifics as they were now fully pledged allies. Speaking of the devil, Gentoku arrives at the cafe with a retro-style outfit and asks if Sento's memories were indeed back. Sento confirms so, and the show comically entered it's commercial break.

The Secret of the Lost Bottles
Sento sits out on the beach by himself and is visited by Takumi's voice inside his head, who confirms that the Lost Bottles were created by Shinobu Katsuragi, Takumi's father, and that Project Build has no data on them due to Evolto getting rid of them before Takumi was able to find out. Sento is then visited by Kazumi and Ryuga. Kazumi wants to head to Hokuto to track down the Lost Bottles, but Ryuga doesn't think it's a good idea and becomes surprised that Sento wants to go as well, believing that if he visits his mother's house in Hokuto, he might find something there. Ryuga bitterly heads back to the cafe, but not before giving Kazumi his Dragon Sclashjelly.

Sento and Kazumi arrive in front of the Katsuragi residence and Sento decides to go in and investigate, while Kazumi wants to walk around thinking about what he's fighting for, now that the Three Crows were gone. Sento walks inside and uncovers a box full of Shinobu's belongings and discovers a photo of Reika Kine. He sends a picture of the picture to Ryuga, who confirmed that she was the one who smuggled Ryuga and Sawa into the country. Sento sends Ryuga to meet her, while Sento headed off to help Grease at Faust Headquaters. Grease reveals that the Lost Smash is actually Hokuto's Prime Minister, Yoshiko Tajimi, and that she will die if defeated in battle. Build fights MadRogue, but he mocks him saying there was no use in going against Evolto, as they were all his puppets. Build retorts as he enters his Genius Form by saying that they weren't puppets and that they were actual humans with souls, ready to sacrifice everything to protect someone. His words encourage a defeated Grease to get back up and fight back against MadRogue. Sento then realized that Build's Genius Form is capable of saving the Lost Smash victims, and used its finishers to knock Yoshiko out of her Smash form and safely neutralize the Hazard Gas in her. With MadRogue surprised of this new development and distracted, Build and Grease are able to defeat MadRogue, and Utsumi retreats. Sento suggests using the Bottle on Kazumi but he declined saying the Gas made him stronger and will be counting on Sento if the worst case scenario happens. The two check up on Yoshiko who bawls in regret of her recent actions, but Kazumi forgives her. Kazumi picked up the Lost Bottle that Yoshiko used, which had apparently turned completely black after Build defeated her with Genius Form. Evolto appears and is surprised that Sento has found a way to counteract his plans and demands for the Lost Bottle back, wanting to create a new world. He asks for Sento's assistance, but he refuses and responds by transforming to Genius Form and attacks Evolto. They fight on an equal standstill, and Evolto pulls out his Evol-Trigger. As Evolto attempts to insert it into his Driver, Build grabs hold of his hand to prevent him from activating it, and attacks Evolto with a Genius Finish, causing Evolto to detransform back into Soichi. Out of respect for Sento's new power, Evolto leaves, deciding to take the Lost Bottle another day. After de-transforming, Sento receives a call from Ryuga who met with Reiko and confirmed that Shinobu Katsuragi was alive.

With everyone gathered at the cafe, Sento said he was already suspicious about his father's death as a body was never recovered. Sawa reveals that Reika helped smuggle Shinobu out of the country to fake his suicide, and Sento becomes angered that his father never told his family. Sawa also reveals a USB-drive that she received from Reika, which as they pulled up on the computer, contained the original schematics for the Rider System, as well as information on the Lost Bottles, which confirmed that not only will they make people aggressive and hostile, but a supernatural phenomenon will happen should 10 of the Bottles be gathered. They begin to wonder as to why Evolto hasn't destroyed the planet already, and if he was trying to gain power, he would have already done so after obtaining his Black Hole form. Reviewing Evolto's actions, he first came to Earth in the form of Ryuga Banjou. 10 years ago, he possessed Soichi Isurugi and failed to raise Pandora's Tower and caused the Skywall Incident instead. Evolto was taken to a mental hospital and escaped, where he met with Shinobu to have his Evol-Driver restored. Several years later, he arrived at a Faust hideout where he obtained his Transteam Gun and Cobra Fullbottle and began working as Blood Stalk, sending the team Smash after Smash, and forcing Misora to purify the Fullbottles they obtain. They then realize that all the Smash that were sent after them were people that were somehow connected with the team, such as: Kasumi Ogura who was Ryuga's girlfriend, Reika Kine who smuggled Shinobu and helped fake his death, Masahiro Nabeshima who framed Ryuga, Tatsuya Kishida, who was friends with Taro Satou, one who had his face swapped with Takumi Katsuragi, Takumi's mother Kyoka Katsuragi, Sento's former co-worker and researcher Shingo Kuwata, and finally Sawa herself. Stalk's identity was later revealed to be Soichi, and war between the three countries for Pandora's Box would begin. Evolto would take advantage of the war to raise our heroes' Hazard Levels and force Sento to make upgrades to the Rider System. Finally, Evolto would regain his Evol-Driver and Kamen Rider Evol would be born. He would possess Ryuga and gain access to Phase 2: Dragon, which he used to open Pandora's Box. Evolto would possess Sento and gain access to Phase 3: Rabbit. Spitting out Sento who lost his memories and essentially becoming Takumi Katsuragi, Evolto gained access to Phase 4. Sento would regain his memories and finally have access to Genius Form. Sento realized that while recapping their adventures, he realized that the first Smash he fought when he broke Ryuga out of prison did not have any sort of apparent connection with the team, and asked Sawa to investigate, while trying to figure out what kind of supernatural phenomenon would happen should the 10 Lost Bottles be gathered together.

Sento and Misora visit the comatose Soichi at the hospital, and Sento brings up that he found out that that all the Best Match combinations were connected to Soichi's memories. The Fullbottles require the energy of the Earth, and Soichi filled 30 of the bottles with things that Misora liked as a child, rabbits, pandas, hedgehogs, etc. He was then asked to name things that could destroy those things Misora liked, and came up with names like tanks and gatling guns, eventually naming harmless things like erasers and manga to prevent defiling memories of his daughter. Misora began to apologize to her father for not understanding his feelings.

Back at the cafe, Sawa reveals the Smash Sento first fought was Kyoichi Shimizu, who was a student of Shinobu at his university. Sawa couldn't find out what he was doing before becoming a Smash, but confirmed he was currently working as a bartender. Sawa had already met with him and agreed to meet Sento. As Sento waited at the location, Takumi asked him why he was so desperate to find their father. Sento responded that after realizing the truth behind the Best Matches, he found that human emotions might be the only way to defeat Evolto, which leads to the reason as to why Shinobu created the Rider System in the first place. Shimizu arrived and immediately transformed into a Lost Smash, and Sento fights him as Build Genius Form. During the battle, Shimizu reveals that if he doesn't defeat Build, Evolto will kill him, but Build assured him he won't as he used a Genius Finish on Shimizu to safely neutralize him out of his Smash form. Build picks up the Lost Bottle and Shimizu told him that to create a black Pandora panel, he created the Lost Bottles. Before he could say anything else, Evolto appeared and killed him, who revealed that he himself has human emotions now and wanted to feel the excitement of seeing Sento's rage. The two fight again, but Build is knocked known. Evolto takes out a Lost Bottle and attempts to transform Build into a Smash, but Genius Form neutralized the Bottle and Build attacks Evolto back. Sento stands firm on his belief that the Lost Bottles were created for justice, and did not waver to Evolto who liked toying with human emotions. Anything that Evolto tried to destroy, Sento vowed to build back. He uses a Genius Finish to knock Evolto down, who responds with rage and attacks Build back in anger. Evolto then picks up the Lost Bottles and leaves, saying he'll make sure Sento drowns in despair.

A New Enemy, His Own Father
Sento returns to the cafe and breaks the news that Evolto now possesses human emotions. Sento tells everyone about the Black Pandora panel, which requires the 10 Bottles, and they deduce that Shinobu might be working with Evolto. Sento looks at the USB files again, but couldn't find any leads to where Shinobu might be or if he was innocent in any way. Kazumi believes Shinobu to be the mastermind of everything they've been through, which angered Sento. The team then receives a call from Touto's government who is requesting help to fight against a Seito invasion. The team heads out. Sento and Gentoku go straight to Evolto, and Sento demands for his father, while Evolto continues to play dumb. Gentoku suggests focusing on saving Touto for now as they both transform into Rogue and Build Genius form respectively. Sento fights Evolto while asking for his father, but Evolto performs a technique that took out Rogue and brought Build to his knees. Evolto summons a Black Hole and sucks up the entire Touto residence office, before leaving.

Back at the cafe, the team watches on TV that Evolto, disguised as Masakuni Mido wishes to "reunite" the country, while scoffing at the impersonator. Even though everyone else is happy the country will be united, the team sulks in despair, knowing the full truth that they're under the control of an evil tyrant.

Sento is relaxing at the rooftops where he is suddenly pulled into the "world of memories" as Takumi called it. Takumi confessed that he had the power to control what Sento can and can't remember. Sento affirmed that he'll find their father and put an end to Evolto, but Takumi is distraught to hear that Sento still believes in their father. Sento reminded him that Takumi completed the Rider System because he at one point, also believed in justice. After leaving the space, Sento returns to reality and is visited by Gentoku, who reminisced of his father and regretted not becoming the son his father wanted him to be, even after his death. Sento felt the same way as he should have defeated Evolto by now. Sento and Gentoku return to the cafe and hear the others watching the news. The newscaster reports that the Kamen Riders have formed an anarchy, and now rebels are wreaking havoc all over Touto. Sento and Gentoku head to the reported area and engages into battle with MadRogue. While fighting the Smash, Build sees Rogue getting defeated by MadRogue and attacks MadRogue, but is defeated too. Utsumi stated that the entire country has become their enemy, so there was no point in fighting anymore. Sento responded that if their future was stolen by Evolto, then they'll have a reason to fight, which meant they couldn't give up for the sake of the people who trust in them. Gentoku responds with the same and affirms that the Kamen Riders will never die out. The two fight back with new resolve and are able to defeat Utsumi, affirming that they will eventually take back their home from Evolto. After de-transforming, the two are then attacked by another Kamen Rider Build, who then reveals himself to be Shinobu Katsuragi, leaving Sento in complete shock. Shinobu noted that while his face wasn't the same anymore, his habits stayed. Suddenly Evolto appears right next to Shinobu, causing Sento to berate his own father about the true purpose of the Rider System. Ryuga and Kazumi arrive and are also just as shocked to hear that Shinobu deceived Sento. Shinobu reveals his true intent to collect the remaining Lost Bottles and transforms into Build NinninComic Form and begins fighting Rogue, Cross-Z, and Grease, while Sento stands behind in bewilderment. Kazumi is easily defeated, and Shinobu reveals that the Build Driver was originally created for his use, and thus he was the best user of the Rider System. He transforms into KaizokuRessya form and defeats Rogue. He then transforms into HawkGatling, but suddenly Ryuga goes wild and easily beat down Shinobu. He continues to punch Shinobu, while Sento tells him to stop. Ryuga refused and was about to kill Shinobu with the full-might of the Magna Knuckle. Sento transforms into Genius Form and stops Cross-Z and detransforms Ryuga, who falls unconscious. Shinobu realized he had enough and flies away.

Back at the base, Ryuga is put in bed and Misora is unable to heal him. Kazumi then angrily yells at Sento for protecting his father and realized he could no longer trust him and leaves, with Gentoku also leaving for the same very reason. Sawa and Misora go back upstairs while Sento takes his frustration out on the computer itself, angrily throwing the monitor to the wall. Takumi told him that Evolto was right all along and that they were nothing but puppets. Shinobu was there the day Pandora's Box rose the Skywall, so it was natural he would eventually side with the alien. Sento said Shinobu should have regained his sanity when he transformed with the Build Driver, but he is unable to as he's the creator of it. Sento sits in regret as Misora comes in, telling him not to blame himself as she would have done the same for Soichi. Sento appreciated those words, and came to the ultimate realization that his father was indeed their enemy. Sento began to wonder what he had been fighting for all this time, but Misora reminded him that Sento Kiryu has always found a way to overcome every single obstacle he ran into.

After some time, Misora left the cafe. However Sawa realized she was gone for too long and told Sento, who then received a phone call from Misora calling for help. He headed to her location and found her groggily walking towards him. Vernage then takes over and demands for her planet back. She pulls out a Lost Bottle and transforms into the CD Smash. Sento transforms into Build TankTank Form and fights her, but is overwhelmed by a barrage of CD attacks. He tries to use the Genius Bottle, but Shinobu appears and tells Sento that Genius Form will not work as Vernage is acting as a barrier from allowing Genius Form to safely neutralize the Hazard Gas inside Misora. Shinobu transforms into Build RabbitTank, and the two charge at Sento. Shinbou rambles about his "new world" and how the Lost Bottles will bring forth a "phenomenon that will transcend the laws of physics", eventually knocking Sento out of his transformation. Sento lies in despair, depressed that his father deceived both him and his mother. Shinobu confirms so and says Sento was a hero fabricated for his plans and will never defeat Evolto. Sento begins to sob, but Ryuga appears telling him not to. He reminded Sento that what they believed in wasn't an illusion, but for true justice. Sento thought back to what Misora said before she left, and with Ryuga's help, got up. Equipping their Build Drivers, they transform into Build Genius and Cross-Z Magma. Ryuga took on Shinobu, while Sento devised of a way to save Misora. Finding the path to victory, Sento charges up a Genius Finish and works with Ryuga to safely safe Misora, while transferring Vernage over to Ryuga instead, which neutralized his episodes. The two work together to take down Shinobu, while Sento is able to steal the Rabbit Fullbottle, reaching Hazard Level 7. Sento affirmed that he'll be better than his father, and defeat Evolto. Shinobu chuckles and activates a Phoenix Fullbottle and escapes.

Back at the base, Sento analyses Ryuga's compatibility with Vernage and gets amazing results, so Ryuga shouldn't have any more episodes and is now stronger than before. Sento reveals the newly created Blizzard Knuckle, which he reveals can defeat Evolto. Sawa comes in saying Gentoku and Kazumi are at Faust's hideout, with their location confirmed via GPS. Sento and Ryuga head to the hideout transformed to rescue them. Ryuga extracts them out while Sento stayed behind to fight Utsumi, who he easily defeats, and his father. Shinobu is unimpressed that Sento is still highly unprepared for fighting Evolto and attacks the Hazard Trigger, which immobilized Sento. Shinobu then covertly helps Sento escape, which leaves Sento confused as to why is father would do that.

Sento returns to the cafe and interrogates Kazumi, but with a small push he pushes Sento across the cafe. He reveals he received a Nebula Gas dosage, but Sento told him he'll die if defeated. Kazumi agreed, but he also said if Evolto wins, then everyone dies anyway, and didn't want to feel like he was holding the team back. Kazumi and Gentoku rejoin the team, and Kazumi asks if Sento has come to a conclusion about his father yet. Sento says he has, and heads downstairs. He sits at the computer and remember what his father said to him during their last battle. Misora comes down and Sento reminds her that they must defeat Evolto as soon as possible, but is hesitant in letting Kazumi and Gentoku fight due to their recent gas dosage. Later on, Sento has Ryuga try to transform using the Blizzard Knucle, but as the failed attempts continue, Sento tries to push him to it. As a result, Ryuga loses consciousness and Sento goes up to the rooftop. He receives a call from Evolto who demands for the Lost Bottles he has. Sento refused, but Evolto threatened to destroy the Earth, so Sento had better listen to his demands. Sento angrily throws his phone on the ground and returns to the cafe, where Gentoku has made a pasta dinner based on Kazumi's recipe. However as Sawa and Misora eat it, they spit it out in disgust, and Kazumi wonders why it was so bad despite being made the same way. That would however trigger an idea in Sento on how to defeat Evolto. If the Hazard Trigger was such a flawed weakness for Build, then the Evol-Trigger should act the same way for Evolto.

A Tyrant's End
That night, Sento sneaks out of the cafe to meet Evolto for the bottle he wanted, but wasn't there to give it to him, but to defeat him. As Sento pulls out the Blizzard Knuckle, the others arrive and blame Sento for taking all the fun. They arrive and tell Sento that they're comrades and they should be fighting together. Sento smiles and the four transform to fight Evolto. However even with their combined might, they were not strong enough and were blasted away with a Black Hole Finish. But Sento affirmed that as long as they worked together, they can turn the impossible into the possible, despair into hope, and defeat into victory. With a new plan in motion, Rogue and Grease hold down Evolto while Cross-Z attacks him with the Blizzard Knuckle. The Knuckle hits the Evol-Trigger, immobilizing Evolto in place, allowing Build to use his Genius Finish and defeat Evolto once and for all. Evolto acknowledges his defeat and as he disappears, he explodes in place.

A Father's Pride
Ryuga and Kazumi cheer, but Sento isn't convinced the battle is over as he finds three Lost Bottles in the aftermath. Suddenly Shinbou flew in with a large container and took the Lost Bottles from Sento. He reveals that it was his plan for the Riders defeat Evolto, and that his genes were stored in his Driver. Shinobu places the Driver inside the container and assures that Evolto while he is still alive, he cannot regenerate. Shinobu has the Riders follow him into the Tower and shows them the Black Pandora panel. He reveals that Evolto lost to them on purpose just to receive human emotions and gain the ability to purify the Lost Bottles. After his death, Shinobu was supposed to revive him, but never expected Shinobu to betray him. He asked Sento to forgive him, but Sento wasn't so quick to do so. If Shinobu was undercover this whole time, then why didn't he stop him sooner? Shinobu revealed that during the Skywall Incident, many people died, and felt the only way to save the world was to create a new one entirely. Once he finishes the Black panel, it will create a wormhole that will allow anyone to travel to and from any planet. When Evolto destroys a planet, he absorbs the planet's life energy for himself. Once the portal was complete, then he could instantly travel to several countless planets and destroy them in only a matter of seconds, gaining more power at a faster rate than he ever could before.

A Tyrant's Return
Suddenly the team is ambushed by Utsumi, who is then revealed to be possessed by Evolto, who is rather unhappy that Shinobu betrayed him. He reveals that before his main body exploded, he was able to extract a small piece of himself out, and it possessed Utsumi. He then without a sweat kills Shinobu. In his dying breath, he gives Sento two data cards on Build and Cross-Z and tells him about a White panel that even Evolto didn't know about and that the Hazard Trigger was the key. Before he disappears, he smiles saying that "Takumi" has grown taller. Sento wallows in despair, giving Evolto a chance to retrieve his Driver from the container. Realizing the imminent threat, Gentoku whisks everyone out.

For almost an entire day, Sento has been typing away on his computer trying to figure out how the Hazard Trigger fits into all this. Takumi then tells him that a white panel does not exist and that Shinobu was not a good person. Sento tells him that Shinobu was undercover this whole time, but Takumi wanted an explanation as to why Shinobu would create the Hazard Trigger then. Sento believes that the Hazard Trigger must have some good purpose and a connection to the White panel. Takumi thinks Sento is too trusting, and does not want to be fooled by their father any longer. Back in reality, Misora brings news that Touto was being attacked by Evolto's Smashes. The Riders arrive and run after Evolto. In a fit of rage to avenge his father, Sento attempts to activate the Genius bottle, but to no avail. Evolto reveals that he's not in his final form because he has possessed Kazumi, who arrived with the final Lost Bottle. He releases himself from Kazumi and pushes him back with the team. With the last Lost Bottle, he transforms into Black Hole Form and inserts the full panel into himself, resulting in a horrifying new monster form. The Riders fight him, but he demonstrates powerful firepower without breaking much of a sweat, knocking Cross-Z, Rogue, and Grease back extremely easily. Evolto takes Build and they warp to a different planet, where he is surprised to see Sento still alive. He forms a black hole that sucked every living thing up before warping him and Build to space, where Build gets a front row seat in seeing the planet get destroyed. They warp back to Earth, and with even more power, Evolto knocks Build across buildings, up to the sky, and down on the ground, disabling Sento out of Build. Evolto then voiced his intent to destroy Earth next, when suddenly Sento remembered something. 10 years ago before the ceremony, Shinobu revealed to Takumi that he was currently researching the existence of aliens, and asked his son if he would protect the Earth with him should an alien invasion happen. Takumi agreed, and Sento is bummed that he didn't remember this memory sooner. Remembering that both him and his father created the Rider System to protect Earth from alien harm, he is able to use the Genius bottle once again and transforms alongside the Hazard Trigger. With the Riders gathered, the four simultaneously attack Evolto with their Rider Kicks, extracting the panel out of Evolto. Build activates a simultaneous Genius-Hazard Finish to purify two of the Lost Bottles on the panel, knocking Evolto back to his Black Hole form, resulting in his eventual retreat. Sento looks at the Hazard Trigger that night and knew that the Hazard Trigger would come through with justice, while reminiscing about his father. He goes back downstairs and figured out a surefire way to end Evolto once and for all.

Build Annihilation Plan
Intercepting a stray Smash threatening a pair of siblings, Sento found himself overwhelmed by a mob of civilians whom had been compelled to attack him. As it turned out, Japan's new Prime Ministers were actually members of the Blood Tribe, members of the same race as Evolto whom had been operating in the shadows up til now; declaring Build an enemy of the state, Touto's new Prime Minister, "Kengo Ino", commenced the Build Annihilation Plan. Sento quickly lost all support as Sawa and Misora fell to mind control, Rogue and Grease were overwhelmed by the horde, while Ryuga was personally brainwashed by "Kengo Ino". Finding himself attacked by Ryuga, Sento was robbed of the Hazard Trigger, which was provided to "Kengo Ino", who used it with Ryuga's own Great Cross-Z Dragon along with a Build Driver and the Cobra Lost Fullbottle to transform into Kamen Rider Blood, combining his power with Ryuga and his fellow Blood Tribe members.

Rescued from Blood's mercy by Evolto, Sento was intent on saving Ryuga. Realizing that a duo of Fullbottles in the Build Driver could allow him and Ryuga to defeat evil together, Sento soon confronted the Blood Tribe and the possessed Ryuga. Though outmatched by Blood, Build was able to land a blow which seperated Ryuga from him, proceeding to clear Ryuga's mind with the power of the Genius Fullbottle. Though faced by the Lost Smash and the compelled civilian masses, Sento and Ryuga were covered by Grease and Rogue. Presenting the Gold Rabbit and Silver Dragon Fullbottles, Sento revealed to Ryuga that they could both use this pair of Fullbottles to fight together. Though Ryuga was skeptical, Misora channeled the power of Vernage to combine the Fullbottles with the Genius Fullbottle, resulting in the creation of the Cross-ZBuild Can, a device which enabled a transformation combining Sento and Ryuga into Kamen Rider Build Cross-ZBuild Form. Ultimately, this new power proved great enough to destroy Kamen Rider Blood, though it reverted back to the Genius Fullbottle shortly after.

Journey to the Top of Pandora's Tower
Evolto finally revealed his true colors and his intent to destroy the Earth publicly on live television, and challenged the Kamen Riders to a final duel at the top of Pandora's Tower, while demanding for the last Lost Bottle in their possession. Sento has Ryuga use his innate Evolto DNA to create the White panel by inserting the Hazard Trigger inside Pandora's Box. Revealing that he got the idea from analyzing his father's research data, the simultaneous use both the black and white panel will not only allow the user to travel to parallel dimensions but to create entirely new worlds by merging two completely different ones, the current world and alternate world with no Sky Wall. Thus this merger giving rise to singularities that not even physics can explain. Sento explains that while the people of Earth have different versions of themselves in another universe, Evolto & Pandora Box does not, so the merger of the worlds will completely erase him and the Pandora Box from existence.

Later that day, Sento asks Takumi for his opinion on the plan, and Takumi felt that even though it was a good plan, too many sacrifices were made for it. Kazumi then comes into Sento's workspace and asks for a power-up, even though they were short on time. Sento reveals that the Blizzard Knuckle was actually originally made for Kazumi, but because Kazumi went ahead and got himself kidnapped at Faust to raise his hazard level, Sento wasn't able to perform the Test. He also tells him that if Kazumi does use the Knuckle along with the new Bottle that goes with it to Transform, his Hazard Level will rise beyond the limits of a normal human, and killing him in the process. Therefore, Sento suggest that Kazumi only use it as a weapon, Kazumi did not care as he felt that risking your life was part of being a Kamen Rider. He was prepared to die as his old three friends were gone, so he had nothing to lose. However Sento told him that if Kazumi died, then the entire team would be sad. Kazumi later rebukes Sento warning as his Sclash Driver only had one slot for Fullbottle, thus Transforming using the Blizzard Knuckle is impossible to start. Misora then came in and invited everyone to the rooftops for a barbecue. The team grill meat, have drinks, and light fireworks (Gentoku alone eat Soba for unknown reason). Ryuga saw Sento looking serious at one point and asked him what was wrong. Sento realized that this seemingly normal life was what they were trying to fight for, and that was why they had to defeat Evolto. He then told Ryuga that in order for the Lost Bottles to synergize with the Black and White panels, it requires Evolto's DNA to do so. Obviously Evolto himself won't be willing to do it, so Ryuga is the only one who can. Sento told Ryuga to do what he can however he can, no matter the cost, even if Sento and the others die in the process.

The next morning, several TV stations have lined up at the doorsteps of Pandora's Tower live to broadcast the "duel" between Evolto and the Kamen Riders. Sento and his team arrives as the mass media run away after Evolto appeared with a huge army of Hard Guardians and after a brief welcome, Evolto explained his rules. As the Kamen Riders attempt to climb up the tower, they will be ambushed by Smashes all around. For every 10 minutes that pass without them having already reached the top, Evolto will destroy a part of the country using a massive Black Hole, then proceeded to demonstrate what he was talking about. The four Riders transform and take out the entire army with a simultaneous attack. They head in the tower, but are met with imitations of the Hokuto Trio, created by Evolto's DNA. Grease decides to stay behind, while Build, Cross-Z, and Rogue attempt to make their way up the tower while fighting the horde of Smash going after them. The three are then stopped by imitations of the Hell Bros, and Sento is forced to allow Rogue to stay behind while him and Ryuga trek their way up. They both watch the video feed of Kazumi fighting as Grease Blizzard, and Ryuga becomes livid at Sento for allowing Kazumi to use the Blizzard Knuckle, and the two have no choice but to keep going. Sometime later at the top of the Tower, while Sento and the other is treating to Gentoku's wounds, Misora arrives with news of Kazumi's death and the purified Castle Lost Bottle. As Sento grips the Lost Bottle that Kazumi sacrificed himself to purify, the entire team continues to wail in sadness. Evolto arrives and greet them light-heartedly, thus making Sento lashes out at the tyrant.

Sacrifices Made
Evolto expresses his "soft love" for Earth after living on the planet for an entire decade, but admits that humans have always been stupid in his time on the planet. He announces that it was due time for the planet to be destroyed and transforms into his Black Hole form. Sento watches Gentoku get up wanting to fight, despite his injuries. Commending his courage, Sento, Ryuga and Gentoku transform into Build Genius, Cross-Z Magma, and Rogue respectively and charge at the evil tyrant. The three gang up on Evolto, with Build Genius matching Evolto in speed, but gets flung across and hits Cross-Z, "dropping" his Hazard Trigger for Ryuga. With the three Riders down, Evolto becomes unimpressed by the performance they've shown. Build rushes at Evolto again, but backs off when Ryuga rushes in after powering himself with the Trigger attached to his Driver and attacks Evolto relentlessly. Build watches as Cross-Z loses and Rogue rushes in to take a chance, but fails and began to disappear slowly. Evolto attacks Build again and makes him drop the Lost Bottles. Evolto inserts the Lost Bottles into his Black Panel, activates his Trigger's Feverflow, inserts the panel into himself, and transforms into his monster form. Evolto warps himself into space and generates a Black Hole that sucked up the entire moon, shocking the entire team into bewilderment. Evolto warps back to Earth and uses his newfound powers to attack Sento and Ryuga, knocking them both out of their transformations. While Misora tends to them, Evolto summons another Black Hole and announces he'll destroy the Earth. The sheer size of the Black Hole turns the sky black and provided Evolto with even more power, and transform Evolto monster form into it's Ultimate State.

As the team wallows in despair, not knowing what to do, Misora asks for Vernage's help to no avail. Seeing their cries as pathetic, Evolto attacks the team, but Rogue shields them from harm and rushes at Evolto. Encouraged to fight by Kazumi the night before, Gentoku was prepared to fight in order to protect the country, even after Sawa plead him to stop. Sento, Ryuga, Misora, and Sawa sadly watch Rogue attempt to beat down Evolto even though he won't stand a chance. As a result, Evolto shatters Rogue's helmet, exposing Gentoku's left eye. Rogue continues to fight back to no avail and about to lost consciousness when suddenly the public outside of Pandora's Tower began to cheer for the Rider's victory. Which leads to Gentoku being able to outmatch Evolto temporarily, surprising him. Gentoku said that his father were right this whole time. The power didn't belong to the ones in power, but the people who give them one, and executes a Crack Up Finish to attack Evolto, but he ultimately falls as Evolto swat him to a pillar. Sento calls out his name as Gentoku transformation is cancelled. He finally decides to leave everything to Sento Kiryu and fades away after pondering if he was a step closer to his late father ideology. As the team wallow in sadness, the public's cries reach the ears of Sento and Ryuga, revitalizing their resolve. Sento says that the Rider System doesn't get stronger by anger or sadness, but by hope and trust, and the two proceed to transform into Build Genius and Cross-Z Magma.

Gentoku sacrifice turns out to not completely in vain, as his relentless attack Immobilized Evolto by the Evol-Trigger being damaged, Build executes a Genius Finish on him, extracting the Black panel out. Sento calls for Ryuga to execute the rest of the plan. Cross-Z rushes to Evolto and slams the white panel with the black one, creating an explosion that knocks both Evolto and Ryuga down, and Sento rushes in and disconnects the tether between Evolto and the panels. Sento grabs the sandwiched panels while Misora grabs Pandora's Box and places it in the middle of the tower. Sento shouts out loud for the creation of the new world and places the panel onto the box. It then causes a huge wave of rainbow-like energy to emit the box into the sky, while extracting the energy from the Genius Fullbottle, thus revert Sento into TankTank form. The energy wave then pulls another Earth into orbit and creates a huge ripple between the two planets. Evolto is furious to see his endgame foiled and Sento assures that Evolto will be thrown into the ripple himself. Sento rushes foward, but Ryuga stops him and disengages his transformation. Ryuga rushed for Evolto and said because he shared his DNA, he'd be the best candidate to stop Evolto, alerting Sento of Ryuga's decision to sacrifice himself. Ryuga thanks Sento for everything before brought Evolto and himself to space and disappearing into the ripple. Sento screams Banjou name as he disappeared.

The New World
As Ryuga plunges with Evolto into the wormhole between the two Earths, Sento uses his RabbitRabbit form to go and save him despite Misora and Takumi initial protest. There, he finds out that Evolto has absorbed Ryuga and the remaining DNA he had, exclaiming that unless Sento defeat him, Sento plan will go to waste as the collision will destroy both planet instead. As the two clash, Sento degrades from his RabbitRabbit Form, Tank-Tank, RabbitTank Sparkling, and finally back to his base form, with Evolto losing some of his power as well. Evolto reminds Sento that Sento heroism all started with him and that would be better off without him, Even states that Sento Kiryu is a fake hero created by him. Sento retorting that the power he and his friends had created will be used to create love and peace in the world. Then, Ryuga calls out to Sento from within Evolto who he's holding back, saying that he and the rest of his friends believe in Sento and that he's their hero. Ryuga resist long enough for Evolto to forcefully releases Ryuga's Silver Dragon Fullbottle. Sento uses that bottle along with his Gold Rabbit Fullbottle, to become Build RabbitDragon Form and uses his Vortex Attack Rider Kick to finally defeat Evolto for good. The intense battle creates a heavy tornado in the gap, with Sento trying to grab onto an unconscious Ryuga. By that time, the two worlds merge together to create an Earth without the Sky Wall, finally bringing peace.

Sento wakes up in the middle of a park where SkyWall first stood, holding on what looks to be a stopwatch with Cross-Z on it (the Cross-Z Ridewatch). Trying to wrap his head around the situation, Takumi Katsuragi points out how Sento created a new Earth, but was surprised to realize that the Genius Fullbottle, along with the white and black panels of the Pandora Box, defied the laws of physics. Knowing that he is no longer needed, Katsuragi says farewell to Sento, noting that it's been fun and that he truly did believe in him as he disappeared. Sento is relieved that the world is back to the way it should, but laments how all the people he's met in the previous world will not remember him or his deeds. He does run into the alternate/New World version of Ryuga, who has black hair and is dating Kasumi, but decides not to have him remember Sento and walks away satisfied. He comes back to Nascita for a cup of coffee, where Soichi and Misora are running the café together. Misora asks him if they've ever met before, much to Sento's surprise, but is interrupted by Soichi who believes him to be Taro Satou from the band Lynks. He is overjoyed to have a celebrity at his establishment but has Misora embarrassed, with Sento smiling at the two. Sento also surprised that Soichi is now able to make good coffee.

Later on, Sento is still sad how none of his friends will recognize him, ironically saying he's the only one who remembers now. However, the original Ryuga manages to find Sento, who is having the same problem and even saw his New World counterpart. Sento realizes that a version of Ryuga with Evolto's DNA wouldn't exist in the New World as well, making them the only two who remember the journey they had as Kamen Riders, pleasing him. They ride off together using Machine Builder, with Sento pointing out Ryuga's fly was down ever since they fought Evolto, ready to make a fresh start in their new lives. As they sat together in a park, Sento decides to chronicle their journey into 49 episode scripts and begins to narrate them, with Ryuga butting in on the narrations.

2017
Due to Uhr creating Kaijin Rider Another Build in the time-stream, it resulted in Another Build's powers coming into conflict with Sento's powers as Build, and later Ryuga Banjo in extension due to the temporal disruptions. After Zi-O gained the Build Armor, the Rider would uses Build's power the defeat the imposter. After Another Build's defeat, Sento and Ryuga lost their memories and Sento reverted to Takumi. Sougo Tokiwa gives "Takumi" and Ryuga each a blank Ridewatch, which Takumi walks off in awe of the device.

2018
"I'll leave my past self to you."

- Sento to Sougo as he re-entrusts the Ridewatches to Sougo

One year later, "Takumi" and Ryuga returned to Nascita after a Tsunagizu concert at the. However when the arrive, Sougo Tokiwa was there as well, but Sento and Ryuga did not recognize him until they pull out their blank Ridewatches (that Sougo gave them the year before), and transformed themselves into the Build and Cross-Z Ridewatches, which would temporarily restore Sento and Ryuga's memories. Remembering the events from last year, Sento entrusts the Ridewatches to Sougo so he can go back in time and defeat Another Build.

Personality
"Me and him...We fought to become strength for others. To protect people...we kept rising back up. Even without you...I still have people to protect. For the justice I believe in...I will defeat you!"

- Sento proving his justice to Blood Stalk. Sento's outstanding traits are his intellect, narcissism, and sense of justice. A genius by all standards, Sento has repeatedly displayed advanced knowledge in physics, mathematics, chemistry, engineering, and cryptography.

Sento's polymathism is accompanied by an inflated ego and sharp tongue: he likes to praise himself for his works, and often mocks Ryuga for his comparatively lower intelligence. Sento often relies on his own skills to solve problems and never expects sudden assistance from others, so when it happens, he tends to either be surprised or turns them down. Also, Sento can be easily caught off guard when he accomplishes anything or witnesses something unexpected and new which excites him, because of his reliance on his intellect. In this state, he is vulnerable to any surprise attacks.

Sento can be easily attracted to anything that is mysterious and interesting, such as new innovations and anything that brings him closer to uncovering his past, and will relentlessly pursue it. This will continue until either he reaches a conclusion or someone forcibly stops him during progress. When Sento gets excited (usually due to a discovery or breakthrough), a part of his hair sticks up.

Sento's greatest trait is his sense of justice. A firm pacifist, he believes that science should be used for the sake of humanity, and harshly opposes using it for war. His selflessness and sense of duty causes him to try and shoulder the burden alone rather than involve others, whether he can carry that burden or not. This altruism intensifies the guilt he feels for his role in creating the Smash and escalating the war. While generally calm and composed, Sento is prone to making poor judgement during emotional turmoil.

Sento has a couple of catchphrases. Before he transforms, he says "Now, shall we begin the experiment?" (さあ、実験を始めようか？). In Kamen Rider Build: Be The One, he says a variant of this catchphrase, "Now, shall we begin the final experiment?" (さあ、最後の実験を始めようか？). After transforming, he says "The rules of victory is set." (勝利の法則は決まった).

Current Powers

 * High Hazard Level Physiology: Sento underwent an experiment from Faust which injected him with a certain amount of Nebula Gas. However, instead of mutating into a Smash, the gas retained his body without side affects apart from memory loss (mostly inflicted by Blood Stalk). As result, his initial Hazard Level starts at Level 3, which allows subjects like him to utilize the Build Driver to transform into Kamen Rider Build. Later, his Hazard level rose to 3.7 due to his anger during the battle against Blood Stalk. After continuous usage of the RabbitTank Sparkling Fullbottle, his Hazard level rose to 3.9; and after continuous use of the Hazard Trigger, his hazard level was at 4.4 according to Blood Stalk, last time it was measured. By using a panel from the Pandora Box in the Build Driver, Sento was able to raise his Hazard Level to 6, a feat that should not be possible for a human being. Soon after he raised it to 6.3+ by cranking the Build Driver's Vortex Lever. During his fight with the CD Lost Smash and Shinobu Katsuragi as another Build, Sento raised his Hazard level to 7.
 * Genius-Level Intellect: Sento's most defining trait is his intelligence. He has an advanced knowledge of physics, and developed several of Build's accessories on his own. Since regaining his memories of Takumi Katsuragi, his intellect has combined with the latter as well.
 * Genius Physicist: Sento has an advanced knowledge of physics, having finished a difficult physics test with perfect results. This proves a significant asset in battle, which Sento incorporates into his fighting technique. Sento also seems to understand the fundamentals of time travel as well, as he is currently the only legacy Rider in Zi-O to remember Sougo giving him the blank Ridewatch and return it, knowing Sougo is a time traveler.
 * Genius Engineer: Sento has built the majority of Build's accessories, as well as all of Cross-Z's equipment, on his own.
 * Strategist: Sento has shown himself to be a capable strategist, being able to outwit Blood Stalk on two occasions as well as Namba.
 * English Language Proficiency: Sento is able to read Shinobu Katsuragi's entire file and perfectly translate it back to Japanese verbally.
 * Fullbottle Augmentation: By shaking a Fullbottle in his hand, regardless of whether he is transformed or not, Sento can gain abilities based on said Fullbottle's traits:
 * Rabbit: Allows Sento to gain fast reflexes. First used to defeat Ryuga Banjo in a one-on-one fight.
 * Gold Rabbit: Allows Sento to perform powerful punches. First used to defeat Shinobu Katsuragi's version of Build.
 * Gorilla: Allows Sento to perform fiery punches. First used to fight Ryuga Banjo.
 * Fourth Wall Interaction: Used only for comical purposes, Sento is capable of interacting with Ryuga's recollection of his birth in various ways. This normally involves said recollection being thrown away or knocked off-screen somehow.
 * Hazard Level Measurement: Sento gained this ability following his acquisition of Genius Form. He was able to detect Evolto's rising Hazard Level while fighting him.

Weaknesses

 * Dragon Fullbottle: As seen in episode 10, Sento's body cannot handle the Dragon Fullbottle's power as he transforms into Build KeyDragon, which causes him to be electrified by the energy surges and experiences intense pain. Further use of the Dragon Bottle causes his body to be engulfed in flame-like energy which hurts him even more and forcibly knocks Sento out of his transformation. This was due to Ryuga's Hazard Level 3 synchronizing more effectively with the Dragon Full Bottle from prolonged use and exposure to its energy, while Sento had never used it before, and hence lacked that immunity.
 * Dragon Jelly: Sento is also incapable of using this item as well when he uses Banjou's Sclash Driver.
 * Form Data: As seen on Utsumi's tablet, Namba has data on most of Build's Best Match forms, including tactical data for countering the. They also have data and counters for RabbitRabbit Form, which they used against Sento in the fight with Rogue. Hypothetically, any of Sento's forms can be countered, provided they're analyzed and the opponent is powerful enough to utilize the counters.
 * Hazard Trigger: If Sento uses the Hazard Trigger for too long, it will overstimulate his nervous system and send him into a mindless rampage, destroying anything and anyone in his path until his transformation is cancelled. Raising Sento's Hazard Level appears to prolong the amount of time he can use the Trigger with his consciousness intact. Sento can overcome the effects of the Hazard Trigger by using it along with the FullFull RabbitTank Bottle. When the Force Ride Meter on the Trigger is struck forcefully with an attack, it will paralyze him for a period of time. This weakness is carried over from the Evol-Trigger, which the Hazard Trigger is based on.
 * Hazard Kill Switch (Formerly): With Utsumi's help, Sento designed a switch that would allow the user to overload the Hazard Trigger should Sento ever lose control while using it. Activating the switch would destroy the Hazard Trigger, killing Sento in the process. However, after creating the FullFull RabbitTank Bottle, it was no longer needed.
 * Negative Emotions: Since Sento's acquisition of Genius Form, the form is entirely based around Sento's heroic traits and will not activate should he lose those traits. The first instance of this happening was when the bottle was first completed and Takumi attempted to use it, and could not since he did not have any trust whatsoever in anyone. The second instance was shortly after Evolto had killed Shinobu Katsuragi, causing Sento to enter a fit of rage that prevented the Genius Bottle from activating.

Devices

 * Build Driver - Transformation device.
 * Fullbottles - Transformation trinkets.
 * Fullbottle Holder - Carrier strap for Build's Fullbottles
 * Hazard Trigger - Build's power up device, used to upgrade into a Hazard Form by attaching it to the Build Driver.
 * Evol-Driver - Evol's transformation device. Used while possessed by Evolto.
 * Evolbottles - Evol's transformation trinkets. Used while possessed by Evolto.
 * Build Phone - Build's personal smartphone that can transform into the Machine Builder.

Weapons
Handheld weapons:
 * Drill Crusher - Build's personal drill-like weapon
 * Hawk Gatlinger - Build's gatling rifle-like weapon accessed by any form that uses the Taka or Gatling Fullbottle.
 * 4Koma Ninpoutou - Build's katana-like weapon accessed by any form that uses the Ninjya or Comic Fullbottle.
 * Kaizoku Hassyar - Build's bladed bow-like weapon accessed by any form that uses the Kaizoku Fullbottle.
 * Fullbottle Buster - Build's personal sword-like and pump-action shotgun-like weapon, accessed by any form that uses the FullFull RabbitTank Bottle.

Suit-equipped weapons:
 * Rabbit Foot Shoes - Build's rabbit-like spring shoe accessed by any form that uses the Rabbit Fullbottle.
 * Tank Roller Shoes - Build's tank-like tread shoe accessed by any form that uses the Tank Fullbottle.
 * Muscle Glove - Build's gorilla-like gauntlet accessed by any form that uses the Gorilla Fullbottle.
 * BLD Prism Glove - Build's diamond-like glove accessed by any form that uses the Diamond Fullbottle.
 * Solstall Wings - Build's hawk-like wings accessed by any form that uses the Taka Fullbottle.
 * Onmitsu Scarf - Build's ninja scarf accessed by any form that uses the Ninjya Fullbottle
 * Giant Scratcher - Build's panda-like gauntlet accessed by any form that uses the Panda Fullbottle.
 * Space Ride Arm - Build's rocket-like missile accessed by any form that uses the Rocket Fullbottle.
 * Spine Knuckle - Build's hedgehog-like tekko accessed by any form that uses the Harinezumi Fullbottle.
 * Multi Deluge Gun - Build's fire engine-like hose accessed by any form that uses the Shoubousha Fullbottle.
 * Gold Lio Gauntlet - Build's lion-like gauntlet accessed by any form that uses the Lion Fullbottle.
 * Long Range Cleaner and Trash Converter - Build's vacuum cleaner-like weapons accessed by any form that uses the Soujiki Fullbottle.
 * Bind Master Key - Build's key-like accessory/batting weapon accessed by any form that uses the Lock Fullbottle.
 * Train Gauntlet - Build's train-like wrist guard accessed by any form that uses the Densya Fullbottle.
 * Twist Lash Arm - Build's octopus-like whips accessed by any form that uses the Octopus Fullbottle.
 * Shade Breaker and BLD Light Bulb Shoulder - Build's light bulb and circuit breaker-like weapons accessed by any form that uses the Light Fullbottle.
 * Flame Reviber and Empirial Wings - Build's Phoenix-like gauntlet and wings accessed by any form that uses the Phoenix Fullbottle
 * Demolition One and BLD Arsenal Shoulder - Build's robot-like claw arm accessed by any form that uses the Robot Fullbottle
 * Wolfatal Claw - Build's wolf-like claw accessed by any form that uses the Wolf Fullbottle
 * Build Pad Shield - Build's Build Phone-like shield accessed by any form that uses the Smapho Fullbottle
 * Ibalash Arm - Build's thorn-like gauntlet accessed by any form that uses the Rose Fullbottle
 * Batrotor Blade - Build's helicoptor-like rotor accessed by any form that uses the Helicopter fullbottle
 * BLD Tora Fight Glove and Jungle Foot Shoes - Build's tiger-like glove and shoe accessed by any form that uses the Tora Fullbottle
 * Inbay Dodge Shoes - Build's UFO-like shoe accessed by any form that uses the UFO Fullbottle
 * Navy Chest Armor and BLD Spout Glove - Build's whale-like chestplate and glove accessed by any form that uses the Kujira Fullbottle
 * HZ Deadly Gloves and HZ Vanish End Shoes - Build's gloves and shoes accessed by any Hazard Form

Vehicles

 * Machine Builder - Build's personal Rider Machine that is transformed from the Build Phone.

Allies/Team Build

 * Misora Isurugi: When Sento first became Kamen Rider Build, he was introduced to Misora by Soichi, and especially her ability to fertilize Fullbottles. Although they usually bicker and prank each other, Sento cares about her greatly and consoled her who felt guilty about being the cause of the current war. It was due to her response that Sento realized that to end this war, he had to get his own hands dirty by fighting as Build, not as a weapon of warfare, but to uphold a sense of justice and protect his friends and his home. Sento is also aware of the importance of Misora's online identity as Mii-tan, and withheld this fact from Kazumi Sawatari when he asked. For the proxy duel, Sento entrusted an empty Fullbottle to Misora that would have the capabilities of stopping Build in his Hazard Form, if he would go out of control. It would not only destroy the Hazard Trigger, but it would also kill Sento as well, Misora had disagreed with this method and simply refused to use it. Luckily when Ryuga appeared and was able to successfully knock Sento out of Hazard Form, Misora cries in joy and didn't use the kill switch. Throughout the war, Misora continues to show her care for Sento as she noticed he hasn't been smiling much, and promised him to fight for "love and peace," an ideal he strongly held on to during his proxy duel with Rogue. Misora would later berate Sento for not taking care of his own life, as he was willing to sacrifice himself for the sake of his friends and family.
 * Ryuga Banjo/Kamen Rider Cross-Z (Charge): Sento refers to Ryuga as his "best match" for helping him search for his memories and Night Rogue, as he's the only surviving victim from the experiments who retained his memory and escaped before entirely becoming a Smash. They are shown to constantly banter, however they both have great trust and innate respect for each other. When Soichi revealed his true plans, Ryuga got angry and attacked Blood Stalk for Sento and Misora's sake. Later on when Sento was feeling depressed after learning of his own true identity, it was Ryuga who encouraged him to accept for what he was and fight as Build again. Sento was initially unaware of how much he has done for Ryuga and despite not expecting anything back, Ryuga still wished to do the same for him. In turn, Sento accepted the consequences of using the Hazard Trigger for Ryuga's sake as well, although he lost his will to fight after he killed someone in his time of berserk, even surrendering his bottles to the Touto government and turning down the offer to participate in the proxy duel. Ryuga was struck with a sense of guilt, blaming himself for forcing Sento to resort to the dangerous Hazard Trigger, and decides to apologize to the Hokuto Three Crows for Sento. During the proxy duel, Sawa informs Ryuga that Sento was going to die after using the Hazard Trigger. Just before Hazard Build was going to kill Kazumi, Cross-Z Charge interfered and was able to successfully knock Sento out of his Hazard Form upon giving him the pep talk. Ryuga was given the credit for saving him, but at the end he said the credit belonged to everyone. Later after realizing the truth of Ryuga's origin and his own father's past, the two continued to work together in order to stop Evolto from using the Pandora Box. When Evolto approached Ryuga with the offer to join him and destroy the world, Ryuga affirmed that he would never help Evolto, acknowledging Sento as his one and only partner. However after Ryuga was absorbed by Evolto, Sento had vowed to protect the Pandora's Box from Evolto's hand no matter what, in honor his friend. Sento went through a great deal to extrapolate Ryuga from Evolto nearly at the cost of his own life. However when Sento was possessed instead, Ryuga vowed to rescue Sento from Evolto. Due to Ryuga relentless and boorish fights, Sento is able to unlock Genius Form. When Sento is knocked down after Misora becomes a Smash and Shinobu having an offensive upper-hand, Ryuga appeared and reminded Sento of the the true justice they were originally fighting for, and to never forget that. With Ryuga's encouragement, Sento is able to safely force Misora out of her Smash form, while transfering Vernage over to Ryuga instead, who used his newfound power to push Shinobu back. During their final battle with Evolto at the top of Pandora's Tower, Sento and Ryuga were the last two remaining warriors left to battle Evolto after Kazumi and Gentoku gave their lives to fight the evil tyrant. Ryuga offered himself as sacrifice to drag Evolto into the singularity ripple as he had Evolto's DNA as well. Ryuga disappeared into the ripple alongside Evolto, creating the new world. When Sento walked around to explore the seemingly new world, he finds a black-haired Ryuga walking alongside his fiance Kasumi, and called out to him in relief. However being a new version of Ryuga, he does not recognize him. Thinking that Sento was one of his MMA fans, he offered a handshake in which Sento accepted in relief. As Sento sits at the fountain, it is revealed that Ryuga survived the singularity and is actually the real Ryuga, who was psyched to see another him with an alive Kasumi (the Ryuga and Kasumi that Sento shook hands with). The two rejoice each other's company as they ride Sento's bike to a park, with Sento pointing out that Ryuga's fly was down. As they sit, Sento reveals to Ryuga that he has digitized their memories into 49 episodes, and they both begin to narrate each episode.
 * Sawa Takigawa: Sawa was saved by Build before he unknowingly and accidentally revealed his identity and led her to his base. In exchange for her silence, she asks that Sento gives her an interview on the Kamen Rider. Sento agreed on the condition she not only kept his identity secret, but also aid him in solving the mystery of the Smash and his missing memories. Eventually, Sento finds Sawa a bit annoying, as she comments wildly when he is trying to battle. After turned into Smash by Night Rogue and saved by Ryuga as Cross-Z, Sento and Ryuga are shocked as Sawa tells them that she's actually a Faust Spy. In the end, she is forgiven after explaining her reasoning. Sawa is shown to have upheld her dignity and sought Soichi's past files on Sento's request, and continues to support them. When Sento went out of control during the proxy duel with Grease, she realized the only person who could save Sento was Ryuga himself. Later when Sento learns about the Nanba children and their undying loyalty for Juzaburo Namba, he questioned Sawa about her father's past, telling her there were no recorded victims with the name "Takigawa", Sawa replies saying Takigawa was her mother's name. Sento had finally completed the FullFull RabbitTank Bottle and it's accompanying gear, and was about to head out, but he stopped to hear Sawa's story. She revealed to him that her father dying in the gas explosion was a lie, and she herself was one of the Nanba Children. Everything from her being turned into a Smash to being kidnapped by Touto was all part of her plan to gain Sento's sympathy and trust, and if her position was ever compromised, she'd have to commit suicide. However her feelings changed after being with Sento and his friends for so long, it gave her a "family" that she never had. She also revealed that Namba had Nabeshima's family hostage, and wanted Build's upgrade schematics in exchange for their freedom. They formulate a plan (that succeeded) to have Sawa give Seito the data to only RabbitRabbit form, while Sento keeps TankTank stored away until Kazumi is able to rescue the Nabeshima family.
 * Kazumi Sawatari/Kamen Rider Grease: Although Kazumi was Sento's enemy during the war, Sento showed sympathy for him since Kazumi was also a war victim. Sento withheld Misora's identity as the online idol Mii-tan from Kazumi. After Sento is struck with grief upon killing Aoba, he begs Kazumi to hit him. However he refuses saying that Aoba's death wasn't his fault and that this is a normal result of war. However because Aoba was a good friend of Kazumi, he swears to defeat him in the proxy battle, should Sento participate. After the duel ended in Grease's defeat, Sento was still under the influence of the Hazard Trigger, and while Ryuga tried to stop him, Kazumi warned Ryuga that it was dangerous. But, after Ryuga succeeded in doing so, Kazumi admit defeat fairly. After Seito made it's invasion into Hokuto, Kazumi and Akaba returned home to save Kiba and their farm, with Sento and Ryuga saying they can't risk joining them due to alliance differences, but they'd wait at the Sky Road. After Kiba's death at hand of Washio Brothers and both Kazumi and Akaba return injured, Sento and Ryuga promise to help them avenge Kiba. Later when Sento is paying his respects to Aoba and Kiba, it was apparent that Kazumi no longer held hard feelings for Sento, and suggested that they team up against Seito. When Build used the Hazard Trigger against Rogue, Grease said he'll knock him out himself if he goes berserk. Since then, Kazumi has been working alongside Sento's team. During the proxy 3-on-3, Kazumi saved the Nabeshima family, thus signaling Build to use TankTank and defeat Rogue. Sento thanked him for that, but Kazumi said the honor was his and that he is now able to return to Hokuto. Although, due of Namba discreetly replacing Mido, Kazumi had no choice but to stay with Team Build while working solo. After Namba was killed by Evolto, Kazumi now focuses on liberating Hokuto, with Sento's assistance. However when it is revealed that Shinobu Katsuragi was working with Evolto against them, Sento would "help" his father, causing Kazumi to get angry and lose his trust in him. Sento would later realize how much of an enemy his father was, and regain both Kazumi and Gentoku's trust. Later when Kazumi asked Sento for a power-up, Sento realized that the Blizzard Knuckle was made for him, but with the way Kazumi is now, it would kill him. Kazumi was prepared to die as he felt that since the death of the Hokuto Trio, he had nothing to lose, but Sento told him that Kazumi did have something to lose at that time, and it was the team and they'd be sad if he died. However during the journey to Pandora's Tower, they were ambushed by copies of the Hokuto Trio, and Kazumi decided to stay behind, which Sento decided to allow. Later when the team are waiting at the Tower's top, Misora arrives with news of Kazumi's death, which greatly saddened the entire team. Sento held onto the North Blizzard Fullbottle and lashed out at Evolto upon seeing him.
 * Soichi Isurugi: 10 years before the start of the series, Soichi arrived on Earth after being possessed by Evolt. At that time, it's not known whenever Takumi/Sento had met Soichi before being possessed. Sento believed that when Soichi was just Soichi, he looked after him and was always "home" when Sento returned. That, however all began to disappear when Soichi was revealed to be Blood Stalk and possessed by an alien. To this day, Sento continues to fight in the justice that Soichi taught him, against Evolto. In the new world, Sento walked into Nascita and asked for a cup of coffee. However as Misora takes his order, Soichi immediately recognized Sento as Taro Satou, the famous lead singer of the band Links. Sento braves himself as he drinks Soichi's coffee, thinking it would be as terrible as before, but is surprised to see how good it was. As Soichi attempts to tell Sento about the blend, Misora stops him and they playfully argue in Sento's eyes.
 * Gentoku Himuro/Night Rogue/Kamen Rider Rogue: Sento trusts Gentoku as a more senior employee. Sento views Night Rogue as a rival, with this hatred is fueled further by how he was responsible for the former's experimentation and loss of his past memories. However when Soichi revealed Gentoku's true identity as Night Rogue, Sento confronts Gentoku and they fight for the Pandora Box. It was Gentoku who revealed Sento's true identity to him, information he received from Soichi. Later, during the war between Touto and Hokuto, Sento and Banjo are forced to work with Gentoku. After Taizan returns as Prime Minister, Sento soundly defeats Gentoku, but he returns as a Kamen Rider Rogue who allied with Seito and takes down both Build and Cross-Z without even breaking a sweat. Now referring to Sento as his original identity Takumi Katsuragi, Gentoku reveals that the ambitions that he fights for are several times stronger than both Build and Cross-Z combined, and that they'd never understand the pain he went through to get where he was right now. Taking their battle to the outskirts of the Skywall, Rogue defeats Hazard Build with ease. Later during the proxy battle between Touto and Seito, Sento battles Rogue in an attempt to break the even match, however his own motives are questioned by Rogue in an attempt to hypo-criticize him. Sento responded that he fully understood how much of the war was caused by Takumi Katsuragi because of his invention of the Rider System, and felt it was in his responsibility as the instigator of the war to end it. After receiving RabbitRabbit Form's data, Gentoku gained the upper hand and continued to ramble on about his ambition and how it was the only thing he felt he could rely on, and quickly rejected Sento's idea of "love and peace". After being outwitted by Sento's revealing of TankTank Form, Sento reaffirms that even though "love and peace" may not be the ideal form of belief in the reality they were currently in, it at least gave him the strength to fight for a new world. With Gentoku fighting Build blind from TankTank Form, Build is able to defeat Rogue by transformation disengagement, and is congratulated by Taizen, who tried to bring Gentoku back to Touto, but he brushed it off and walked away, leaving Sento in worry. Gentoku later reveals to Sento in the midst of battle that due to a chip implanted in him, he is forced to obey Seito's commands, and reveals the entire Prime Minister situation to him. He asks Sento to take back the Pandora Box to prevent Juzaburo Namba from taking over the entire world. Later when Taizan Himuro is kidnapped, Gentoku calls Sento to help him disable the chip in him to help him stop Evolto and save his father. Ultimately, After his father demise at hand of Evol Cobra, Gentoku now works alongside Team Build. Sento however was shocked and rendered speechless after seeing Gentoku's gaudiness, especially when it comes to his retro style of causal fashion. After Sento "helps" Shinobu by preventing Ryuga from killing him, Gentoku realizes that Sento is too desperate to save his own father, and feels they cannot fight alongside each other anymore. Later, Gentoku along with Kazumi rejoined fighting alongside Sento after raising their Hazard Levels and discovering the truth behind Shinobu's plan with Evolto. Since then, Gentoku has been a great asset to the team, with Sento allowing Gentoku to stay behind and fight the imitated Hell Bros. When Gentoku fought Evolto to the death, he uttered "Sento Kiryu"'s name for the first time, signifying a sign of respect that Gentoku has developed for not only Takumi, but for Sento himself as well. Luckily before Gentoku died, he was able to damage Evolto's Evol-Trigger, allowing for Sento and Ryuga to execute their plan and drive Evolto into the singularity created by the power of the Black and White panels.
 * Takumi Katsuragi: Sento's relationship with Takumi is a complicated one, since they're both in fact one-and-the-same. Sento has heard of the name on numerous occasions, and is in possession of the Project Build files Takumi left behind. Sento has heard of horrible things about Takumi, and was both distraught and depressed to learn he was Takumi himself. However after some encouragement from Ryuga, Sento is able to mentally separate himself from his past self and be just Sento Kiryu, and only Sento Kiryu. Although, he still didn't want to militarize the Rider System, as he is still partly at fault in the war between Touto, Hokuto & Seito. After Gentoku began activities as Kamen Rider Rogue, he began calling Sento, Katsuragi, acknowledging the existence of his former co-worker as the same as Sento. Sento realizes all the horrible things that he has done as Katsuragi, and took up the the responsibilities of righting all his wrongs. Later when it's revealed that his father was in charge of experimenting on Ryuga as a child, Sento is forced to look back at Takumi's (Sento's) past. When Evolto demanded the Evol-Driver in Sento's possession, Sento was able to track down the location of the Driver that he as Takumi hid. Later when Evolto ejected Sento out, Sento's memories were of Takumi's instead. Takumi had a hard time fighting Evolto in RabbitRabbit form, as he was not used to the high-performance specs on Build, unlike Sento. Takumi adjusted RabbitRabbit's specs to fit his own, and was able to defeat several Clone Smashes. To fight MadRogue, he tries to use the newly made Genius Fullbottle but was unable to do so due to him not believing in his friends. Later with some encouragement from Ryuga, Takumi appears to Sento, asking him if he truly believed in the power of friendship, and asked the latter to prove it and show the future that friendship will build to him, assimilating their memories (albeit Sento is in control) and thus unlocking Genius Form. Later when Sento is thinking about the Lost Bottles, Takumi is somehow able to speak into Sento's mind, saying that it was their father that created the Lost Bottles. During the final battle with Evolto, after Ryuga drags himself alongside Evolto into the ripple, Sento wants to go retrieve him, but Takumi had initially forbade him from doing so, but after hearing Sento's true feelings, he realized that Sento's foolishness was what would eventually save the world. After they arrive in the new world, Takumi says that their existence is an anomaly, since their existence was solely because of Evolto, who did not exist in the world. However Takumi willingly let himself disappear, allowing Sento to live on as just Sento. When Sougo Tokiwa arrives and changes the timeline, Sento's mental identity switches over to Takumi completely, and him and Ryuga live on their lives as non-Riders. Later in 2018, when Sougo arrives for the first time, Sento comes back for a split moment to give Sougo the Ridewatch before switching back to Takumi after Sougo's departure.
 * Sougo Tokiwa/Kamen Rider Zi-O: Sento first meets Sougo when the young boy arrived in his timeline after a shuffle with Geiz. Sougo claimed he was from the future, and Sento confirmed it after coming out of his laboratory and investigating Sougo's Ridewatch. Later when Another Build began to attack, Sougo returned as a Kamen Rider and mysteriously had a portion of Kamen Rider Build's power, allowing him access to Zi-O Build Armor. Sento watches as Zi-O battles Another Build, later becoming unimpressed of the equations that appeared as Zi-O performed his Vortex Time Break. After Another Build's defeat, Sento became Takumi Katsuragi and both him and Ryuga had lost their memories as Kamen Riders. Sougo then gives both Sento and Ryuga each a blank Ridewatch, and asks them to hold on to it as it'll connect to him in the future, but they soon lose their memories of Sougo as well. One year later, Takumi and Ryuga return to Nascita after a concert by Taro Satou, only to find Sougo in the cafe. With Sougo's presence, the Ridewatches that Sougo (from the future) gave them, reacted and transformed themselves into the Build and Cross-Z Ridewatches. Regaining his memories of what happened last year, Sento smiled, knowing that this Sougo wasn't the same as the one he had already met, and entrusted him with the two Ridewatches, asking him to take care of his past self. Sougo would return to the past and repeat the events that Sento had already remembered, resulting in an endless timeloop.

Enemies

 * Evolto/Blood Stalk/Kamen Rider Evol: When Sento was Takumi before, often Evolto (under the guise of Soichi Isurugi) would frequently show up at Faust to help them further their experiments. However, when Takumi wanted to quit, Blood Stalk "killed" Takumi and "made" him into Sento, where he was "found" and "first taken in" from an alleyway. At that point, Evolt believed that Sento had the potential to become Build. Stalk views Build as an equal to Night Rogue but take his words and assistance into account. He believes that science will one day bring the end of mankind, but Sento disagreed. Before being aware that Soichi was Stalk, Sento's relationship with the shop owner was on very good terms. However, upon realizing the two were one and the same, Sento didn't want to believe that the Soichi he knew was dead. However, Sento lost all reason to forgive Soichi after hearing the latter's story and vowed to defeat him to upload that sense of justice Soichi had previously taught Sento to believe in, in honor of the old Soichi. Despite this, Soichi continues to call Sento to check up on both him and Ryuga. Later, Soichi gives Sento the Hazard Trigger and Fullbottles from Hokuto and Seito, along with their Pandora Panels. Despite all of this, Sento was greatly angered for having his pacifist views shattered. Ultimately, Stalk reprimands him to fight for Touto, for the sake of peoples he cares. After Rogue appears, Blood Stalk reappeared to Sento once again and began to compare the two, much to Sento's anger. As Blood Stalk continues to get enveloped in the Pandora Box's powers, he attacks Sento and the Riders and lures them into a labyrinth where everything is in the Box's control. Blood Stalk was willing to sacrifice Sento for the sake of making Ryuga stronger. Later he called Sento after kidnapping Taizan Himuro to exchange his life for the Evol-Driver in Sento's possession. Unfortunately, Sento's further plans failed as Evolto manages to possess Ryuga, causing Sento to fight Evolto on a more personal level. Eventually, when Sento decided to take Evolto down with him, they both survive and Evolto is able to possess Sento due to him reaching Hazard Level 6. Thanks to Ryuga relentless charge, Sento was able to reject Evolto, albeit with memories of Takumi Katsuragi instead of Sento Kiryu. When Takumi completes the Genius Fullbottle and Sento uses it against Evolto, Build is able to strategically stop Evolto from using the Evol Driver and defeat him before he went into his Black Hole Form. Out of respect for Build's newest invention, Evolto decided to halt his hunt for the Lost Bottles.
 * Nariaki Utsumi/Kamen Rider MadRogue: Sento used to look Utsumi as a superior regarding of Pandora Box research, but Utsumi no-nonsense attitude makes Sento think about him as a Cyborg comedically. Before Utsumi's 'Death' at the hand of Gentoku, Utsumi told Sento that he already knows about Sento's alter ego as Build and warns him to not follow his path as a fellow scientist. After Utsumi was saved by Evolto and leaving Namba after going insane, Utsumi was tasked by Evolto to record Build's performance using Hokuto's and Seito's Fullbottles in order to raise his hazard level for the upcoming Proxy battle against Grease, but Utsumi also records Build's performance to perfect the Kaiser system for Namba's usage. To this point, Sento found Utsumi rather incomprehensible. After Utsumi went insane and became Evolto's ally as MadRogue, Sento finally saw Utsumi as an enemy.
 * Katsuragi Shinobu/Kamen Rider Build (Another): Initially, Sento as Katsuragi Takumi held fond memories of his father before the Sky Wall incident. Sento also strongly believes that Shinobu creating the Rider System to defeat Evolto, despite Kazumi and Takumi warnings. However, after the man himself showing that Rider System isn't created for such pretty words, Sento fell into despair as what he initially believes was crushed in his very eyes. Thanks to Ryuga support, Sento manage to fight properly as Build and swore to surpass his dad and defeat Evolto. Later, Sento learned the truth behind Shinobu's allegiance to Evolto, though reluctant to accept and forgive him for what he's put his family through. Shinbou then sacrificed himself for his son, dying by Evolto's hands after hearing of his deception.

Others

 * Emu Hojo/Kamen Rider Ex-Aid: It's not known why Sento planned to attack Emu and take his essence. However, later it was shown that the Build who took Ex-Aid essence was none other than his alter ego, Takumi Katsuragi. In Heisei Generations FINAL, Sento later gives Emu back his powers by handing him the Doctor & Game Fullbottles.
 * Kuroto Dan/Kamen Rider Genm: Apparently, Sento attacks him for taking the essence. However, he declares that he was not Ex-Aid, due in fact that Genm was black, while Ex-Aid was originally pink. Once again, it was his alter ego who attacked Genm.
 * Parado/Kamen Rider Para-DX: In Heisei Generations FINAL, Parado encountered Sento into his homeworld and demands Sento to give Emu his powers back. Sento later remembers Parado as the red and blue Kamen Rider from their previous encounter in Kamen Rider Ex-Aid: True Ending.
 * Tatsuya Kishida: Before Taro had disappeared, Tatsuya revealed that he and Taro were childhood friends and bandmates. They were so close that they called each other "bros." Initially, it was believed that Sento was, in fact, Taro, and despite "having no memory of being Taro", Sento protected Tatsuya as per his duties as Build.
 * Taro Satou: Although Sento has never met Taro, he has heard of him from Taro's friend Tatsuya. When Sento was Takumi, Takumi called Taro in for a drug test, but this was all prevented by Blood Stalk. Stalk then used the murdered Taro's appearance to change Takumi into Sento.
 * Taizan Himuro: Taizan is on good terms with Sento, having trusted him that he'll win in the proxy duel with Grease and Rogue. Taizan would later entrust Sento in planning out the attacks against Seito, shortly before his death.

Portrayal
Sento Kiryu is portrayed by Atsuhiro Inukai (犬飼 貴丈), who also portrays Taro Satou. As Takumi Katsuragi, he was portrayed by Yukiaki Kiyama (木山 廉彬). Under the possession of Evolto, he is voiced by Tetsuo Kanao (金尾 哲夫).

As Kamen Rider Build, his suit actor were Seiji Takaiwa (高岩 成ニ) and Yuya Nawata (縄田 雄哉) in his televised debut in the 44th episode of Kamen Rider Ex-Aid. In the last four episodes of the series, and the first two episodes of Kamen Rider Zi-O, his suit actor is Yuji Nakata (中田 裕士). Under the possession of Evolto as Kamen Rider Evol, his suit actor is Kazuya Okada (岡田 和也).

Rider No.
As the leading rider protagonist of Kamen Rider Build, Build is labelled Kamen Rider #34 (仮面ライダー34号) and Heisei Rider #19 (平成ライダー19号).

Etymology
According to Soichi Isurugi, the kanji used to spell Sento's first name, Sento (戦兎), is composed of the first kanji for tank (戦車) and the kanji for rabbit (兎); this is referring to Build's primary form, RabbitTank. The last name was provided by the name of the barber shop Soichi had been headed out to by the time he "found" Sento.

Appearances
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