イロヨリドリ

Makoto Hirata is a pessimist who struggles to connect with people. His best relationship is with his childhood playmate, Mizuki. One day, by chance, Makoto witnesses the school "playboy" Hasegawa having sex with a girl classmate. Discovering a kind of thrill in the act of watching, Makoto continues to spy on him.
Before long, Hasegawa has moved in on all the girls Makoto knew and cared about... and finally, with the only person he could call his friend.

[From DLsite English]