Tenshi no Inai 12-gatsu

TenInai is about a senior high school boy with relationship problems; he's got few friends and perhaps because of his bothersome kid sister, is averse to any steady relationship. That is, until he has a sexual encounter with Kurihara Touko. Touko, a classmate of his, is a sensitive, simple girl desperate to make an impression on others, and willingly offers herself to the protagonist. He runs away...