Eita Kawai is a member of the Touto Institute of Advanced Matter Physics who is secretly an agent of Faust and an antagonist in Kamen Rider Build.
A researcher at the Touto Institute of Advanced Matter Physics, Eita Kawai was a colleague of Sento Kiryu and secret member of Faust who later took part in Faust's theft of the Pandora Box.
He allowed Blood Stalk to turn him into the Press Smash Hazard so he could hold off Ryuga Banjou. After being defeated by Ryuga, Kawai was reverted to his normal state by Blood Stalk with Cobra Steam Break. He accused Blood Stalk of being Takumi Katsuragi, the "Devil's Scientist", to which Blood Stalk responded to by changing his face to look like Shingo Kuwata's and rendering him unconscious.