G-Senjou no Neko (2002)

Blurb:
Riya Narukawa is a high school freshman with a tremendous talent for the violin and an equally large bag of mental issues. After injuring his own hand to compromise his playing ability, he winds up on the doorstep of college student Atsushi Ikeda, a nice guy who’s maybe too nice and generous. He takes Riya in for the night and helps him with his hand injury. Atsushi realizes there is much more to Riya than is on the surface, and learns just how difficult Riya’s ilife actually is. Not only does he have a personality disorder, but there’s an aggressive senior at his school who has his eye on him. Atsushi becomes a sort of safe haven for Riya, an escape from the life of music he’s been bound to.

Source: www.anisearch.com/manga/4371