The main characters are Okamoto and Koizumi. Okamoto is an athlete, more preciously a basketball player. He just entered his high school just to be with his girlfriend, Tsugumi.
They actually have a story between them. Tsugumi has a large ugly scar on her back. Okamoto accidentally dropped a boiling kettle on her back when they were elementary kids. Since then Tsugumi kind of threatened Okamoto to be responsible for what he had done: being Tsugumi's man. Even though it does not show clear, they even have sexual relationships regularly.
All that was about to change when Koizumi joined basketball club. Koizumi is short, and skinny. That's general description of his appearance. He is actually very cute and feminine. When you see him wearing casual clothes (in near end), you won't be able to tell whether he is a male or a female. And amazingly enough, Koizumi also seems to wish that he was born as a female.