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 Smashes, 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.

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 due to 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 (due mainly to Stalk's interference), Sento and Ryuga snuck to Hokuto to speak with Kyoka.

to be completed

Blood Stalk Revealed
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 and Sento decided to infuse it with the Best Match from a pair of 10 pairs existence after researching on how to use it, resulting on Rabbit Tank which would be infused after RocketPanda, GorillaDiamond, and LionVacuum 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 in aid after distracting Misora by sudden dating distraction plan from Sento. After Ryuga had lost instead, Isurugi told to Sento and Ryuga that Kamen Rider project is nothing more than weaponary project, not for becoming hero of justice and Sento, Misora, and Ryuga were only pawns for fruitation. Angering Sento, he used Rabbit Tank Sparkling and defeated Isurugi. 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.

Sento Learns of his Past
Sento learned a secret access on Takumi's hidden data thanked to Gentoku for telling the password after they fought. Making Sento learned the new upgrade named Sclash project and finished it. Unbestknown to Sento, this had been set by Isurugi as he sneakily came in his cafe and copying the data when he left.

When Gentoku faced and lost against Rabbit Tank Sparkling, Gentoku told Sento that he is Takumi and explained how everything had been set by Isurugi.

Overcoming his Fear
To be added

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 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 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, 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 or Comic 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.
 * 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.
 * Fullbottle Buster - Build's personal sword-like and pump-action shotgun-like weapon, accessed by any form that uses the Full Full RabbitTank Bottle.

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 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.
 * 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. 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.
 * 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. Sawa is shown to have upheld her dignity and sought Soichi's past files on Sento's request.


 * Enemies:
 * Gentoku Himuro / Night 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.


 * Past Riders:
 * Emu Hojo: It's not known why Sento (in that moment Takumi Katsuragi) planned to attack Emu and take his essence.


 * 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.
 * Takumi Katsuragi: Sento's relationship with Takumi is a complicated one, since they've both 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.

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.