Sento Kiryu

Sento Kiryu (桐生 戦兎 Kiryū Sento) 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 Kamen Raidā Birudo) 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 Satō Tarō), his true identity turned out to be Takumi Katsuragi.

History
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 Systems.

Katsuragi eventually had a change of heart and decided to quit Faust. Soichi Isurugi confronted him as Stalk, only for Katsuragi to reply that he had hidden his research data to expose the group. At this time, a man named Taro Satou arrived at Katsuragi's home to apply for a drug-testing job. Knocking Katsuragi out, Stalk then killed Satou before switching the two mens' faces. 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 (the first person to do so), 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.

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.

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.That One with the Best Match

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.

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.

Personality
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.

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

Forms
TBA

Trivia

 * 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？ Sā, jikken o hajimeyou ka?). After transforming, he says "The law of victory is set." (勝利の法則は決まった Shōri no hōsoku wa kimatta).
 * Sento as a Rider makes a variety of callbacks to previous Riders in an attempt by Toei to bring the franchise back to its roots:
 * Build's design uses more basic Kamen Rider design cues than many of his recent predecessors, such as large compound eyes, antenna-like features, and his unique motifs being less prominent in his main form.
 * Build's functionality and design is extremely similar to Kamen Rider Double, the Rider who kicked off the second phase of the Heisei Era, using two trinkets in his black and red Driver to interchange halves of his Forms. Other similarities include form names that combine the names of the trinkets in use, and being the first Rider since Double to not have a unique term for his Forms.
 * Build RabbitTank's color scheme is reversibly similar to Para-DX Perfect Knockout Gamer Level 99, which uses blue and red color connection.
 * Sento's imprisonment and experimentation on by Faust is a reference to the Shocker experimentation origins of the Showa Era Riders.
 * Sento's Ninjya Fullbottle creates a half form that gives him a scarf, making him the first main protagonist Rider since Double to have that classic design element.
 * His Driver is also similar to that of Kamen Rider OOO, as there is a set number of collectible items required for a form, certain combinations match to create powerful forms, and said forms have special jingles, while mismatched forms only state the names of the collectibles.