Kanojo ni Naru Hi

Baka description:

Miyoshi is always competing and losing against his childhood friend, Mamiya, who is good at everything. One day, Mamiya collapses from illness and is hospitalized for several weeks. When he returns to school, it turns out he is genetically female, and has decided to try living his life as a girl now much to Miyoshi’s h*rror

Have fun!

Other names:

变成那个她
彼女になる日
變成女友的那一天
Becoming a Girl Day