Gensei Kihara

Kihara Gensei is an enigmatic character appearing in Toaru Kagaku no Railgun. A member of the Kihara family, he is a man obsessed with Level 6. And he has made several attempts to achieve it.

As the creator of the Ability Body Crystal, his actions would influence the beginning of Toaru Kagaku no Railgun, and several characters in the later part of Toaru Majutsu no Index. For those reasons, he can be arguably considered the main antagonist of the Railgun side story.

(Source: Toaru Majutsu no Index Wikia)