Sento Kiryu

Sento Kiryu is the title character and main protagonist of Kamen Rider Build. He 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 Faust organization and their Smash monsters.

A man with no memory of his past, he was initially believed to be someone named "Taro Satou" (佐藤 太郎 Satō Tarō), but his true identity turned out to be Takumi Katsuragi (葛城 巧 Katsuragi Takumi), a scientist at the Touto Institute of Advanced Matter Physics and the co-founder of Faust. As the war for Pandora Box breaks out between the three countries, Sento accepts reality for what it is in order end the war that his previous identity had started.

Past
Takumi Katsuragi was originally the son of Shinobu Katsuragi, a researcher at the Institute of Space Engineering Technology who supposedly killed himself six months after the Pandora Box triggered the Skywall Disaster that separated Japan.

Katsuragi eventually rose to be genius scientist himself and a researcher at Touto Institute of Advanced Matter Physics, heading the research of the Pandora Box. who was "murdered" by upon discovering his true identity. Seeking a way to carry out illegal human experimentation, Katsuragi co-founded Faust with Night Rogue and Blood Stalk. He would go on to create the Build Driver, the Fullbottles, the Sclash Driver, the Sclashjellies and the Transteam system.

However, Katsuragi later discovered both Night Rogue and Blood Stalk's true identities, as well as Stalk/Evolt's plans for the Pandora Box and connection to Ryuga Banjou. Katsuragi plotted to stop Evolt, luring Banjou to his house so he could kill him. However, before he could do so he was knocked out by Stalk. Stalk then swapped Katsuragi's identity with a man named Taro Satou, wiped his memories and killed Satou so he could frame Ryuga Banjou for it.

After being experimented on by Faust, Katsuragi was placed in an alleyway by Stalk, who then reverted to his civilian identity of Soichi Isurugi and "found" Katsuragi, giving him the new name of "Sento Kiryu".

As Sento Kiryu
Setting up shop in the basement of Isurugi's cafe "nascita", Sento continued to conduct experiments with the Fullbottles and used the Build Driver to become Kamen Rider Build to defend Touto from the Smash monsters. He soon came across Ryuga Banjou, now an escaped fugitive after having been framed for Katsuragi's murder. Believing Ryuga when he said he didn't do it, Build helped him escape from the Touto Guardians pursuing him.

Sento Kiryu initially believed himself to be Taro Satou after he was recognized by a friend of Satou's, Tatsuya Kishida. However, he later learns from Gentoku Himuro that he was in fact Takumi Katsuragi, having had his face swapped with Taro Satou's by Blood Stalk.