Takumi Katsuragi

Takumi Katsuragi, nicknamed by his colleagues as "The Devil's Scientist", is one of the three co-founders of the Faust organization and a minor antagonist in Kamen Rider Build. He is later revealed to be the original identity of the protagonist, Sento Kiryu aka Kamen Rider Build.

History
Takumi Katsuragi was originally one of the top researchers at the Touto Institute of Advanced Matter Physics and the one in charge of studying the Pandora Box. He later co-founded Faust] alongside Night Rogue and Blood Stalk as a way to secretly carry out human experimentation.

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, but failed and 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.

Stalk later reverted to his civilian identity of Soichi Isurugi and "found" Katsuragi, giving him the new name of "Sento Kiryu".