Onigase Tane

A strong (her fists can produce shockwaves) but sweet, kind girl who lives in Chouji's shoe closet. Because of her hair color and abnormal strength she's mistaken for a delinquent by all students except Chouji and other 3 main heroines. Her infamy of being a badass delinquent landed her a couple of (almost) nameless followers like Delinquent A and Delinquent B. Nevertheless, Tane is dedicated only to Chouji, who seems to be the only person out there who's able to recognize her in any costume she wears.

Spoiler

Tane's real surname is 'Sanjouin'. The real 'Onigase' is actually Chouji (it's an abbreviation of his nickname from childhood - 'Oni no Yanagise'). They met each other during their childhoods when he drove off some bullies that were harassing Tane because of her hair color. 'Onigase' taught Tane self-defense and they parted after he staged a fight to bestow his title upon her. After some time, they reunited again with each other in high school.