Sento Kiryu

Sento Kiryu (桐生 戦兎), born Taro Satou (佐藤 太郎), 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.

History
Born Taro Satou, he was friends with Tatsuya Kishida during middle school and formed a band called Lynks with him in their adulthood. However on September 5, as Taro left for a job offer at Takumi Katsuragi's apartment, he was presumably kidnapped by Faust, after which Blood Stalk was deemed responsible for his memory loss, with a single memory left behind which detailed the experiments done on him by Night Rogue. Taro later escaped with no memory of his past and was found lying down in a strange alleyway by Soichi Isurugi, the owner of t he café  Nascita. With no identification, he was given the name Sento Kiryu and taken into Isurugi's care, hidden in the secret basement laboratory inside Nascita.

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 was finally forced 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 Banjyou, 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 due to a Smash alert (a false trail to lure Build out).

Sento returned to discover Ryuga escaped, later discovering Ryuga's fiancé, Kasumi to have also been abducted by the scientists as well. Ryuga choked him into unconsciousness from getting him involved, 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
TBA

Sento Learns of his Past
TBA

Other Appearances
Seeking a sample of Ex-Aid's essence, Build mistakenly intercepted Kamen Rider Genm while he was fighting through the Bugster Virus hordes of Gamedeus Cronus. Switching to the GorillaMond Form to engage him, only after eliminating one of Genm's lives did Build realize that he was not Ex-Aid and promptly took his leave after identifying himself to Dan. Inspired by this encounter, Dan created a Rider Gashat based on Build, the latest in his collection of those of the Heisei Riders, which Emu Hojo snatched from his hand and activate it.

As Taro Sato
Not much to be known of his past life, although according to Tatsuya, he is hinted as a rebellious man who has a large dream of living a luxurious life should their band Lynks become successful. His former apartment implied Taro as a man who lacked a sense of hygiene. Despite all these negative traits, he is nevertheless kind to his friends due to his job application in Katsuragi's lab stems from the need to provide Tatsuya with financial support.

As Sento Kiryu
"You know, in spite of how it may seem, I'm actually a pacifist."

- Build to Genm

Sento is a highly intelligent and sarcastic young man: He likes to praise himself for his works, and has a somewhat child-like curiosity when learning new things, such as when he first sees the Pandora Box. Additionally, when he gets excited, a part of his hair sticks up. On the other hand, Sento is generally composed and calm while fighting.

But when Sento is not composed, he immediately rushes to settle things as seen when he finally met Night Rogue, even telling that he didn't care to know his name when Night Rogue introduced himself, and attacking Night Rogue carelessly. However, rage also enabled Sento to bring his ability as Build to higher potential, shown when he single-handedly managed to injure Blood Stalk. Blood Stalk comments that both Sento and Banjou were exposed to the same Nebula Gas amount to turn people as Smash, thereby classified them as Hazard.

Despite his eccentricity, Sento is a honorable hero. During his job as a Kamen Rider, he always upholds heroism without expecting anything in return.

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 laws of victory have been decided." (勝利の法則は決まった).

Powers and Abilities

 * Hazard Level 3 Physiology: Sento has undergone 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 aside from memory loss. As result, his initial Hazard Level starts at Level 3, where subjects like him can utilize the Build Driver to transform into Kamen Rider Build.
 * 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, if not all, of Build's accessories on his own. Some of Cross-Z's gear like the Cross-Z Dragon was also built by Sento.
 * 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, due to 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 does not have a full control of the Dragon Fullbottle 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 transformation. This was due to Ryuga's Hazard Level 3 better synchronizing with the Dragon Full Bottle from prolonged use and exposure to its energy, while Sento had never used it before and lacked that immunity.

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.

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 all Best Matches is called the Vortex Finish (ボルテックフィニッシュ).

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

 * Drill Crusher - Build's personal drill-like weapon.
 * 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 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.
 * 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 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.
 * Kaizoku Hassyar - Build's bladed bow-like weapon accessed by any form that uses the Kaizoku Fullbottle.

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): Would presumably allow Kamen Rider Ex-Aid to transform into Build Gamer.

Relationships

 * nascita coffee
 * 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 vowed to defeat him and uphold his sense of justice he always had believed in.
 * Misora Isurugi: Sento always uses her for fertilizing Fullbottles, likely being introduced to her ability by Soichi when he suggested Sento become Kamen Rider Build. Although they usually bicker and prank each other, Sento cares about her greatly.
 * Ryuga Banjou: Ryuga is titled as Sento's "best match" by Sento for helping him search for his memory and Night Rogue, as he's the only surviving experiment who retained his memory and escaped before entirely becoming a Smash. They are shown to constantly banter, however they do trust each other.
 * Sawa Takigawa: Sawa was saved by Build before he unknowingly 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, after Sawa explained her reason to become spy, Sento forgives her.


 * Past Riders:
 * Emu Hojo: It's unknown why Sento planned to attack Emu and take his essence.


 * Others:
 * Tatsuya Kishida: Before Sento's disappearance, he was childhood friends with Tatsuya and were bandmates. They were so close that they called each other "bros." After Sento became Build and reunited with Tatsuya, he had no memory of him but continued to protect him as his duties of Build.
 * Enemies:
 * Night Rogue: 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. Sento is unaware of Gentoku's true identity and trusts him as a more senior employee.

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
The kanji used to spell his 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. In-universe, he was given this name by Soichi Isurugi for this reason, who also took Sento's last name, Kiryu, from the name of the barber he had been heading to at the time he found Sento.

His real name, Taro Satou (佐藤 太郎), is likely referring to the names of two subjects related to "rabbit" and "tank" in real life. Taro is derived from the EH500 Kintaro Tank Freight Train and Satou is derived from a Japanese picture book, Sato the Rabbit (うさぎのさとうくん).