Sokubaku Kanojo no Sanjuusou ~Hen'ai: Juuai: Saai no Hate ni~ by Noir Soft - Adult Sex Game

On a certain Valentine's Day, Shouya makes acquaintance with Shizuka. Apparently, her one-sided crush didn't accept her chocolate. While comforting her, he was given the chocolate instead.

Thus, they start to get along. However, Shizuka's love graduatelly becomes obsessive. Furthermore, there is also his friend who has a bizzare love for him, and a lunatic girl who believes him to be her destined partner.

And so, Shouya's life changes greatly, all starts from that moment when he received the chocolate...