Kyou, Furyo wo Hirou

What appeared in front of a college students Tsumugi's apartment was a boy, who ran away from his home. Tsumugi got intrigued by the boy who called himself Kou, who is a relative of someone who once killed his father, what will happen next, that the relative got revealed... ---I regret to death, I wish I had never offered him to stay over that day...