Yuri na Watashi to Akuma na Kanojo(?)

As a child, Yashiro Nonoyama developed an intense fear of men due to boys constantly bullying her. Now in high school, she dreams of being reunited with her first love: a girl who protected her one day from her bullies. Yashiro is in disbelief when a transfer student named Mikoto Saionji arrives who looks a lot like her first love, but she later discovers that Mikoto is actually a guy, and that he was her first love. Yashiro learns that Mikoto is rapidly turning into a girl, and the only way to counteract this change is for them to passionately make out with each other every day.

(Source: Kiss Manga)