Buddy!!!

The very tall Ryuudo Kagetora aims to be the best basketball player in Japan! He has chosen a formerly famous basketball high school, in order to bring it to new glory. He also dislikes girls, ever since a girl beat him at basketball in grade school. There is also another new ace player joining the basketball team, Ichinomiya Chiaki. However, Chiaki was an ace on the middle school women's basketball team. She becomes a man when she's on the basketball court, and she was kicked out of women's basketball team when her quirk is revealed. Ryudo learns her secret, and reluctantly becomes her ally and roommate! (Source: MangaHelpers)