Sento Kiryu

Sento Kiryu (桐生 戦兎) is a genius theoretical physicist who experiments with Full Bottles 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
Little is known about Sento as he has no memory of his past, with the exception of a single memory where he was being experimented on by some scientists using gas masks lead by a mysterious bat-like man.

Somehow Sento escaped, and was found outside the café Nascita by its owner Soichi Isurugi. Isurugi then took Sento into his care, hiding him in the secret basement laboratory inside Nascita. At some point with his genius intellect intact, Sento created the Build Driver and purified Full Bottles, which he used to become Kamen Rider Build. With his new powers, Sento began fighting the Smash, hoping to find answers to his past.

Meeting Ryuga
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 Rouge
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.

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.

Personality
"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.

Despite his eccentricity, Sento is a honorable hero. During his job as a Kamen Rider, he always uphold heroism without expecting nothing 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

 * Genius-Level Intellect: Sento's most defining trait is his intelligence. He has an advanced knowledge of physics, and developed almost all of Build's accessories on his own.
 * Genius Physicist: Sento's has an advanced knowledge of physics, having finished a difficult physics test with perfect results. This proves a significant asset in battle, as Sento incorporates into his fighting technique.
 * Genius Engineer: Sento has built the majority of (if not all of it) Build's accessories on his own.
 * Full Bottle Augmentation: By using a Full Bottle in his hand without transforming, Sento can gain abilities based on said Full Bottle's traits:
 * Rabbit: Allows Sento to gain fast reflexes. First used to defeat Ryuga Banjyou in a one-on-one fight.

Forms
Sento sets two different Full Bottles 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 Full Bottles. However, each one has a Best Match (ベストマッチ) with another Full Bottle that creates an especially powerful form. Best Matches usually have unique names in comparison to other mismatched forms, and the caps of Full Bottles indicate what Full Bottle is its Best Match.

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

Devices

 * Build Driver - Transformation device.
 * Full Bottles - Transformation trinkets.
 * Build Phone - Build's personal smartphone that can transform into the Machine Builder.

Weapons

 * Drill Crusher - Build's personal drill-like weapon.
 * Muscle Glove - Build's gorilla gauntlet in conjunction with any form that uses the Gorilla Full Bottle.
 * Solstall Wings - Build HawkGatling Form's hawk-like wings that grants him flying capabilities.
 * Hawk Gatlinger - Build's rifle-like weapon in conjunction with any form that uses the Gatling Full Bottle.
 * 4 Koma Ninpogatana - Build NinninComic Form's personal weapon.
 * Spine Knuckle - Build's spiked gauntlet in conjunction with any form that uses the Harinezumi Full Bottle.
 * Long Range Cleaner and Trash Converter - Build's vacuum cleaner-like weapons in conjunction with any form that uses the Soujiki Full Bottle.
 * Kaizoku Hassha - Build KaizokuRessha Form's personal weapon.

Vehicles

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

Rider Gashat
Kamen Rider Build Gashat (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: For unknown reason, Sento was helped by Soichi during their first encounter. Both are on good terms like best friend.
 * Misora Isurugi: Sento always uses her for fertilizing Full Bottle, it's not yet known why Sento noticed her power which led Sento to become Kamen Rider Build. Although they usually bickers and pranks 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.
 * Sawa Takigawa: Sawa was saved by Build and he unknowingly reveals his identity in front of her despite the dark night. 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 find Sawa a bit annoying since she comments wildly when sento is on action.
 * Others:
 * Emu Hojo: It's unknown why Sento planned to attack Emu and take his essence.

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 name, Sento (戦兎), is composed of the first kanji for tank (戦車) and the kanji for rabbit (兎).