Ikemen-Kun To Saenai-Kun

Kentarou is a plain, nondescript college student who has never had a girlfriend or any friends. After getting hit on the head by a wayward ball, he begins to notice that there are increasingly long periods of time in which he cannot remember what he was doing. One afternoon, he regains consciousness in an apartment he'd never been to before - and sitting next to him is a young man who then tells him "You're being possessed by the spirit of my dead girlfriend"...