He Is a High School Girl

The delinquent, Bae Choi Kang, is feared as the strongest fighter in his neighborhood. He witnesses his younger sister bullied by a group of girls and soon makes a decision to enter her school secretly to protect her. However, his sister attends an all-girls school. The only way to do this is for him to disguise as a girl. Now forced into all kinds of trouble, he not only has to protect his sister, but also his secret identity from other threats.

(Source: Spottoon)