Kiryu Sento

Kiryu Sento (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. A man with no memory of his past, he was initially believed to be someone named Sato Taro (佐藤 太郎), but his true identity turned out to be someone else. As war for the Pandora Box breaks out between the three countries, Sento accepts reality for what it is in order to end the war that his previous identity had started.

War against Evolto
Sento and Kazumi are called out to an attack at Touto by Seito, where Evolto leads several Hard Guardians to attack the public. Build as RabbitRabbit Form attacks Evolto with a concession of FullBottle attacks where he is able to overwhelm Evol, but he changes to his more gracile Evol Dragon, and Sento responds with TankTank Form. The two exchange a conversation, where Evol thinks humanity is at fault for this war, but Sento believes the humans have the power to prevent it. At the brink of defeat, Build brings out the Pandora Box FullBottle, and affirms that it's for the best, to protect his friends and end this war. He sets the power-up onto his Driver and begins cranking the lever. Evol approaches him but is knocked back after revealing that Build is now at Hazard Level 6. Evol is pleased as he reveals that the plan was going well. Evol strikes his hand inside Sento and releases Ryuga. At that moment, Build began to crank the lever once more, continuously raising his Hazard Level. Evol knows that if Build continues to do this, he will die, but Sento was prepared to take Evolto with him. The two explode, leaving behind the incomplete Evol Trigger, the Evol-Driver, and the EvolBottles.

Personality
Sento's outstanding traits are his intellect, narcissism, and sense of justice. A genius by all standards, Sento has repeatedly displayed advanced knowledge in physics, mathematics, chemistry, engineering, and cryptography.

Sento can be easily attracted to anything that is mysterious and interesting, such as new innovations and anything that brings him closer to uncovering his past, and will relentlessly pursue it. This will continue until either he reaches a conclusion or someone forcibly stops him during progress. When Sento gets excited (usually due to a discovery or breakthrough), a part of his hair sticks up.

Sento's greatest trait is his sense of justice. A firm pacifist, he believes that science should be used for the sake of humanity, and harshly opposes using it for war. His selflessness and sense of duty causes him to try and shoulder the burden alone rather than involve others, whether he can carry that burden or not. This altruism intensifies the guilt he feels for his role in creating the Smash and escalating the war. While generally calm and composed, Sento is prone to making poor judgement during emotional turmoil.

Powers

 * High Hazard Level Physiology: Sento, with the use of the Pandora Box FullBottle, was able to achieve a Hazard Level of 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, the highest documented Hazard Level as of his death.
 * 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 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 Shinobu Katsuragi's entire file and perfectly translate it back to Japanese verbally.

Weaknesses

 * Hazard Trigger: If Sento uses the Hazard Trigger for too long, it will overstimulate his nervous system and send him into a mindless rampage, destroying anything and anyone in his path until his transformation is cancelled. Raising Sento's Hazard Level appears to prolong the amount of time he can use the Trigger with his consciousness intact. Sento can overcome the effects of the Hazard Trigger by using it along with the FullFull RabbitTank Bottle.

Forms
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Devices

 * Build Driver: Transformation device.
 * FullBottles: Transformation trinkets.
 * FullBottle Holder: Carrier strap for Build's FullBottles.
 * Hazard Trigger: Build's power up device, used to upgrade any form into a Hazard Form by attaching it to the Build Driver.
 * Build Phone - Build's personal smartphone that can transform into the Machine Builder.

Weapons

 * Drill Crusher: Build's personal drill-like weapon.
 * FullBottle Buster: Build's personal sword-like and pump-action shotgun-like weapon, accessed by any form that uses the FullFull RabbitTank Bottle.

Vehicles

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

Earth

 * Banjou Ryuga/Kamen Rider Cross-Z (Great/Charge)
 * Sawatari Kazumi/Kamen Rider Grease
 * Himuro Gentoku/Kamen Rider Rogue

Remnant

 * Ruby Rose/Kamen Rider Build
 * Weiss Schnee/