Onimai: I'm Now Your Sister!

Mahiro Oyama, a self-styled “home security guard,” has spent years shut away in his room, passing the time with erotic visual novels and avoiding the outside world. His brilliant younger sister, Mihari, can’t ignore what that isolation is doing to him—and decides it’s time to intervene.

Her solution is as drastic as it is unexpected: a homemade medicine that alters Mahiro’s body into that of a girl. Horrified and unwillingly thrust into a new daily life, Mahiro is left to navigate the world from a completely different perspective, waiting to see whether the change will ever wear off.