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. As the war for Pandora Box breaks out between the three countries, Sento accepts reality for what it is in order to end the war that his previous identity had started.

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 wrestler Ryuga Banjou 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 destroy 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 it's 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, 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, Katsuragi had his memory wiped by Stalk, and was injected with Nebula Gas.

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 Banjou, 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

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 Rygua 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 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 Bi-Kaiser 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 Remecon 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.

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？). After transforming, he says "The law of victory is set." (勝利の法則は決まった).

Powers and Abilities

 * 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.
 * 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.
 * 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.
 * 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 him to be a capable strategist, being able to outwit even Blood Stalk on two occasions and even 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 using a Fullbottle in his hand without transforming, Sento can gain abilities based on said Fullbottle's traits:
 * Rabbit: Allows Sento to gain fast reflexes. First used to defeat Ryuga Banjou in a one-on-one fight.
 * Gorilla: Allows Sento to perform fiery punches. First used to fight Ryuga Banjou.
 * Fourth Wall Interaction: Used only for comical purposes, Sento is capable of interacting with Ryuga's recollection of his birth in various ways. Normally involving said recollection being thrown away or knocked off-screen somehow.

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.
 * Hazard Trigger: (Previously) 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 time he can use the Trigger with his consciousness intact. Sento is later able to overcome the effects of the Hazard Trigger by using it along with the FullFull RabbitTank Bottle.
 * Hazard Kill Switch: (Previously) 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.
 * Form Data: As seen on Utsumi's tablet, Namba has data on most of Build's Best Match forms, including tactical data for countering them, as well as data/tactical counters on RabbitRabbit form to use against Sento as shown 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 counter(s).

Forms
Sento sets two different Fullbottles into the Build Driver and turns the crank to transform into Kamen Rider Build. Kamen Rider Build, like Kamen Riders Double and OOO before him, can assume a variety of different forms, called Trial Forms (トライアルフォーム), using different combinations of Fullbottles. However, each one has a Best Match (ベストマッチ) with another Fullbottle that creates an especially powerful form. Best Matches usually have unique names in comparison to other mismatched forms, and the caps of Fullbottles indicate what Fullbottle is its Best Match. The additional use of the Hazard Trigger upgrades a Best Match into a Super Best Match (スーパベストマッチ).

Build initiates a finishing attack by turning the Build Driver's Vortex Crank again. The finisher of all Trial Forms is called the Vortex Attack (ボルテックアタック), while the finisher of the Best Matches is called the Vortex Finish (ボルテックフィニッシュ). The finisher of the Super Best Matches is called the Hazard Finish (ハザードフィニッシュ).

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 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.

Rider Gashat
Kamen Rider Build (仮面ライダービルドガシャット): Though it was used by Kamen Riders Brave, Snipe, Lazer Turbo and Genm for a yet to be known purpose and both Kuroto Dan and Emu Hojo also used this gashat in their Gashacon Breaker against Nebula Bugsters in Kamen Rider Heisei Generations FINAL: Build & Ex-Aid with Legend Riders, there does not seem to be any indication of a Legend Rider form associated with this Gashat.

Allies

 * 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 Banjou/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.
 * 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 Nascita members while working solo.
 * 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.
 * 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 Banjou 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.

Enemies

 * Evolto/Blood Stalk/Kamen Rider Evol: When Sento was Takumi before, often Evolt (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 takes 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.
 * Nariaki Utsumi: 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 'Death' at the hand of Gentoku, Utsumi tell Sento that he already knows Sento alter ego as Build and warns him to not follow his path as a fellow scientist. After Utsumi was saved by Evolto and working at Namba, Utsumi was tasked by Evolto to record Build's performance using Hokuto and Seito 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.

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.
 * 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 and Hokuto. 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.
 * 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).

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. 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.