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, 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.

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.

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

- Build to Genm Sento is generally composed, albeit somewhat sarcastic and cocky. He likes to praise himself for his works, and has a somewhat child-like curiosity when learning new things, such as when he sees Pandora Box for the first time. When he gets excited, a part of his hair goes up like the rabbit ear from his RabbitTank form as Build.

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.
 * 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 and sometimes provides a unique weapon and able him to perform a finisher. 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. Notably, he is the first Kamen Rider since Agito to have certain parts of his body possess different amounts of power.

Devices

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

Weapons

 * Drill Crusher - Build's personal drill-like weapon.
 * 4 Koma Ninpogatana - Build NinninComic Form's personal weapon.
 * Hawk Gatlinger - Build HawkGatling Form's personal weapon.
 * Kaizoku Hassha - Build KaizokuRessha Form's personal weapon.

Vehicles

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

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.

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).