Otoko Tomodachi Girl

Etou Shougo is a university student sharing an apartment with his male childhood friend Yoshizawa Natsuki. When Natsuki leaves to take part in a clinical trial Shougo didn't know he would be coming back as a girl! It looks like Natsuki won't turn back into a guy for a whole year. Rather than worrying about it pointlessly, why not have some fun with it instead?

(Source: MangaDex)