Sento Kiryu/Zi-O

In 2017, Sento Kiryu (桐生 戦兎) was Kamen Rider Build (仮面ライダービルド), until the creation of Another Build erased the Rider's history, reverting him back to Takumi Katsuragi (葛城 巧).

2017
Due to Heure creating Kaijin Rider Another Build in the time-stream, it resulted in Another Build's powers coming into conflict with Sento's powers as Build, and later Ryuga Banjo in extension due to the temporal disruptions. After Zi-O gained the Build Armor, the Rider would uses Build's power the defeat the imposter. After Another Build's defeat, Sento and Ryuga lost their memories and Sento reverted to Takumi. Sougo Tokiwa gives "Takumi" and Ryuga each a blank Ridewatch, which Takumi walks off in awe of the device.

2018
"I'll leave my past self to you."

- Sento to Sougo as he re-entrusts the Ridewatches to Sougo One year later, "Takumi" and Ryuga returned to Nascita after a Tsunagizu concert at the. However when they arrive, Sougo Tokiwa was there as well, but Sento and Ryuga did not recognize him until they pull out their blank Ridewatches (that Sougo gave them the year before), and transformed themselves into the Build and Cross-Z Ridewatches, which would temporarily restore Sento and Ryuga's memories. Remembering the events from last year, Sento entrusts the Ridewatches to Sougo so he can go back in time and defeat Another Build.

Personality
To be added.

Powers and Abilities

 * High Hazard Level Physiology: Sento underwent 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 apart from memory loss (mostly inflicted by Blood Stalk). As result, his initial Hazard Level starts at Level 3, which allows subjects like him to utilize the Build Driver to transform into Kamen Rider Build. Later, his Hazard level rose to 3.7 due to his anger during the battle against Blood Stalk. After continuous usage of the RabbitTank Sparkling Fullbottle, his Hazard level rose to 3.9; and after continuous use of the Hazard Trigger, his hazard level was at 4.4 according to Blood Stalk, last time it was measured. By using a panel from the Pandora Box in the Build Driver, Sento was able to raise his Hazard Level to 6, a feat that should not be possible for a human being. Soon after he raised it to 6.3+ by cranking the Build Driver's Vortex Lever. During his fight with the CD Lost Smash and Shinobu Katsuragi as another Build, Sento raised his Hazard level to 7.
 * 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. Since regaining his memories of Takumi Katsuragi, his intellect has combined with the latter as well.
 * 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. Sento also seems to understand the fundamentals of time travel as well, as he is currently the only legacy Rider in Zi-O to remember Sougo giving him the blank Ridewatch and return it, knowing Sougo is a time traveler.
 * Genius Engineer: Sento has built the majority of Build's accessories, as well as all of Cross-Z's equipment, on his own. On occasion, Sento has also created gear for both Grease and Rogue, particularly the Grease Blizzard Knuckle and the Prime Rogue Fullbottle.
 * Strategist: Sento has shown himself to be a capable strategist, being able to outwit Blood Stalk on two occasions as well as Namba.
 * English Language Proficiency: Sento is able to read files written in English entirely and perfectly translate it back to Japanese verbally.
 * Fullbottle Augmentation: By shaking a Fullbottle in his hand, regardless of whether he is transformed or not, Sento can gain abilities based on said Fullbottle's traits:
 * Rabbit: Allows Sento to gain fast reflexes. First used to defeat Ryuga Banjo in a one-on-one fight.
 * Gorilla: Allows Sento to perform fiery punches. First used to fight Ryuga Banjo.

Weaknesses

 * Dragon Fullbottle: As seen in episode 10, Sento's body cannot handle the Dragon Fullbottle's power 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 his transformation. This was due to Ryuga's Hazard Level 3 synchronizing more effectively with the Dragon Full Bottle from prolonged use and exposure to its energy, while Sento had never used it before, and hence lacked that immunity.

Forms
 RabbitTank Form "Rabbit! Tank! Best Match! Are you ready? (Techno rock music) Hagane no Moonsault! RabbitTank! Yeahhh!"

- RabbitTank's transformation announcement

Rider Statistics Ability Parameters: RabbitTank Form (ラビットタンクフォーム) is Build's red and blue primary and default Best Match form form accessed from the Rabbit and Tank Fullbottles, hence known as the Moonsault of Steel (鋼のムーンサルト). It is a Touto Best Match, and is themed after a rabbit and a tank.
 * Rider Height: 196 cm.
 * Rider Weight: 99 kg.
 * Punching Power: 9.9 t. (right arm)/17 t. (left arm)
 * Kicking Power: 23.7 t. (right leg)/17.8 t. (left leg)
 * Maximum Jump Height: 55 m.
 * Maximum Running Speed: 100 m. per 2.9 sec.

Being Build's most balanced (and second fastest) form, RabbitTank is the Best Match form favored by Sento, despite being able to transform with any Fullbottles. The Rabbit and Tank Fullbottles are noted as having a particularly high synergy with each other and compatibility with various mechanisms, thus RabbitTank is the basis of several upgraded forms Build can access.

In this Best Match form, Build is armed with the Tank Roller Shoes (タンクローラーシューズ) on his right leg for kicking attacks, and can also act as a working tread to roll along on a flat surface.

This form's weapon is the Drill Crusher. In the final episode, Build uses the Fullbottle Buster while in this form.

As a Best Match, this form has two finishers:
 * Build Driver finisher:
 * Vortex Finish (ボルテックフィニッシュ): Build dives underground to send a white energy chart flying up to the surface with the dotted line securing the enemy before leaping back up, and delivering a kick whilst riding the chart's line. According to Blood Stalk, this finisher has a Hazard Level of 3.2, and then 3.7 when Build attacks him out of anger.
 * Drill Crusher finisher:
 * Vortex Break (ボルテックブレイク):
 * Blade Mode: Build channels the energy of a Fullbottle in his Drill Crusher and performs a powerful slash.
 * Biotic:
 * Rabbit: The Drill Spiral Blade spins such that it creates an energy whirlpool before Build performs a red energy slash.
 * Gorilla: The Drill Spiral Blade cloaks itself in an energy projection that bears a striking resemblance to the Muscle Glove.
 * Ninjya: Purple energy Drill Spiral Blades are projected and spin violently around the Drill Crusher.
 * Abiotic:
 * Rocket: The Drill Crusher charges at the enemy and attacks like a rocket.
 * Gun Mode: Build channels the energy of a Fullbottle in his Drill Crusher and performs a powerful blast.
 * Harinezumi: Build shoots white energy spikes at the enemy.
 * Fullbottle Buster finisher:
 * Just Match Break (ジャストマッチブレイク): Build channels the power of two Fullbottles for a powerful attack.
 * Buster Blade Mode
 * Gold Rabbit + Silver Dragon: Build jumps into the air and performs a powerful downward slash covered in gold and silver energy, followed by two horizontal slashes.

Appearances: Zi-O Episodes 1-2

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.

Weapons
Handheld weapons:
 * Drill Crusher - Build's personal drill-like weapon

Suit-equipped weapons:
 * 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.

Vehicles

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

Relationships

 * Sougo Tokiwa/Kamen Rider Zi-O: Sento first meets Sougo when the young boy arrived in his timeline after a shuffle with Geiz. Sougo claimed he was from the future, and Sento confirmed it after coming out of his laboratory and investigating Sougo's Ridewatch. Later when Another Build began to attack, Sougo returned as a Kamen Rider and mysteriously had a portion of Kamen Rider Build's power, allowing him access to Zi-O Build Armor. Sento watches as Zi-O battles Another Build, later becoming unimpressed of the equations that appeared as Zi-O performed his Vortex Time Break. After Another Build's defeat, Sento became Takumi Katsuragi and both him and Ryuga had lost their memories as Kamen Riders. Sougo then gives both Sento and Ryuga each a blank Ridewatch, and asks them to hold on to it as it'll connect to him in the future, but they soon lose their memories of Sougo as well. One year later, Takumi and Ryuga return to Nascita after a concert by Taro Satou, only to find Sougo in the cafe. With Sougo's presence, the Ridewatches that Sougo (from the future) gave them, reacted and transformed themselves into the Build and Cross-Z Ridewatches. Regaining his memories of what happened last year, Sento smiled, knowing that this Sougo wasn't the same as the one he had already met, and entrusted him with the two Ridewatches, asking him to take care of his past self. Sougo would return to the past and repeat the events that Sento had already remembered, resulting in an endless timeloop.

Portrayal
Sento Kiryu, as well as Takumi Katsuragi, is portrayed by Atsuhiro Inukai (犬飼 貴丈).

As Kamen Rider Build, his suit actor is Yuji Nakata (中田 裕士).