Fujiko Etou

Fujiko Etou is the current head of the Etou family, the head of the girl's dorms at Constant Academy, and the only person who understands Akuto's current situation. Her older brother, the Etou's previous head, was killed after failing a mission, causing Fujiko to become the new head of the family, though for unexplained reasons, she kept her brother's (revived) head, which is kept in a jar-like container filled with fluid. She is the leader of an underground movement that uses black magick (magic) for evil purposes and plans to use Akuto's demon powers once he becomes Demon Lord for her own purposes.

(Source: Wikipedia)