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. He is a man with no memory of his past, and fights to find the truth about himself. Initially believed to be someone named Taro Satou (佐藤 太郎), his true identity turned out to be someone else.

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.

Katsuragi eventually had a change of heart and decided to quit Faust. Soichi Isurugi confronted him as Blood Stalk, only for Katsuragi to reply that he had hidden his research data to expose the group. 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 a Build Driver and a set of Fullbottles that Soichi "stole" from Faust. During Sento's first fight against a Smash, he had fought by-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 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, but was put to a pause because of a Smash alert (a false trail to lure Build out).

Sento returned to discover Ryuga escaped, later discovering Ryuga's fiancé, Kasumi had been abducted as well. Ryuga choked him into unconsciousness to prevent his involvement, but managed to catch up and was too late seeing that Kasumi was already transformed into Burn Smash. Then Night Rogue appeared, where Sento fought to demand answers for his experimentation, but responded that he cared less about every human subject his affiliation experimented. Night Rogue disappeared after delivering the bad news: Kasumi's body was too weak to be restored, and removing her Smash essence would be fatal. Realizing that Kasumi cannot be saved despite Ryuga's pleas, Sento calculated the only way to end her suffering by separating her consciousness long enough for her and Ryuga to say their goodbyes. Ryuga broke down with grief with nothing left to fight for until Sento reminded him Kasumi's life will be for nothing if he gave up and surrender. Sento returned to the lab where Ryuga followed thanked him for the latter, and deciding to cooperate in discovering the truth behind the scientists.

Mirage Smash and Blood Stalk
With the help of Sawa Takigawa, a journalist who had deduced Build's identity, the group determined the identity of the man who set up Ryuga and attacked him in prison: Masahiro Nabeshima. Pausing the search to take 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 sucks. 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 enforces 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
"I will stop you...even if I lose myself!"

- Sento 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.

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.

The night before the duel, Sento had created a kill switch for the Hazard Trigger and asked Misora to use it when Sento goes out of control.

More to come...

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. 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

 * Hazard Level 3 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 Rabbit Tank Sparkling Fullbottle, his Hazard level rose to 3.9.
 * 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.
 * 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.

Weaknesses

 * Narcissism: Sento has overwhelming confidence in his own skills, so he tends to be proud of his own achievements and look down on others (especially anyone who relies on his inventions), which contributes to his prideful personality. 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.
 * Desperate Curiosity: Sento can be easily attracted to anything that is mysterious and interesting and tends to do a total discovery for it, such as new innovations and anything that brings him closer to uncovering his past. This will continue until either he reaches a conclusion or someone forcibly stops him during progress.
 * 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 incapable of using this item as it sends painful electric shocks through his body after being inserted into the Sclash Driver. Later, it was revealed that Sento's Hazard level is lacking as the minimum Hazard Level to use the Sclash Driver is a 4.0.
 * Hazard Trigger: If Sento uses the Hazard Trigger for too long, it will overstimulate his nervous system and sending 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 conscious intact.
 * Hazard Kill Switch: 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.

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
 * Build Phone - Build's personal smartphone that can transform into the Machine Builder.
 * Hazard Trigger - Build's power up device, used to upgrade any Best Match Form into a Hazard Form by attaching it to the Build Driver.

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 Full Full RabbitTank Bottle.
 * Form-based weapons
 * Rabbit Foot Shoes - Build's rabbit-like spring shoe accessed by any form that uses the Rabbit Fullbottle and in RabbitTank Sparkling Form.
 * Tank Roller Shoes - Build's tank-like tread shoe accessed by any form that uses the Tank Fullbottle and in RabbitTank Sparkling Form.
 * Muscle Glove - Build's gorilla-like tekko 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.
 * 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 weapons accessed by any form that uses the Phoenix Fullbottle.
 * Demolition One and BLD Arsenal Shoulder - Build's robot-like weapons accessed by any form that uses the Robot Fullbottle.
 * Wolfatal Claw - Build's wolf-like claws accessed by any form that uses the Wolf Fullbottle.
 * Build Pad Shield - Build's smartphone-like shield accessed by any form using the Smapho Fullbottle.
 * Ibalash Arm - Build's rose-like gauntlet accessed by any form using the Rose Fullbottle.
 * Batrotor Blade - Build's helicopter rotor-like staff accessed by any form using the Helicopter Fullbottle.
 * HZ Deadly Gloves - Build's gloves in any of the Hazard Forms.
 * HZ Vanish End Shoe - Build's boots in any of the Hazard Forms.

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 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.

Relationships

 * Allies
 * nascita
 * 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 dity 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.
 * 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.
 * 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.
 * 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. 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 and both Kazumi and Akaba return injured, Sento and Ryuga promise to help them avenge Kiba.


 * Enemies:
 * 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 allied with Seito and takes down both Build and Cross-Z without even breaking a sweat.
 * Soichi Isurugi/Blood Stalk: Sento was taken in by Soichi during their first encounter, as Soichi believed that Sento had the potential to become Build. Stalk views Build as 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 Seito, for the sake of peoples he cares.


 * Past Riders:
 * 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.
 * 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: To be added.


 * Others:
 * 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 Takumi 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.
 * Taizan Himuro: Taizan is in good terms with Sento, having entrusted him that he'll win in the proxy duel with Grease.

Portrayal
Sento Kiryu is portrayed by Atsuhiro Inukai (犬飼 貴丈). As Kamen Rider Build, his suit actor is 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.

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 Soichi had been headed out to by the time he "found" Sento.