Sora no Seibun

The main characters are Okamoto and Koizumi. Okamoto is an athlete, specifically a basketball player. He joined his high school primarily to be with his girlfriend, Tsugumi. There's a significant backstory between them. When they were in elementary school, Okamoto accidentally spilled boiling water from a kettle onto Tsugumi's back, leaving her with a large, unsightly scar. Since then, Tsugumi has essentially demanded that Okamoto take responsibility for his actions, leading him to become her "man." Despite not being overtly shown, their relationship includes regular sexual encounters. Everything begins to shift when Koizumi joins the basketball club. Koizumi is short and slender, with a very cute and feminine appearance. In fact, when seen in casual clothes later on, it's hard to determine if Koizumi is male or female. Remarkably, Koizumi also seems to wish he had been born as a female. (Source: JanimeS)