Yuuto is the only living descendant of the Amakawa family, one of the 12 demon slayer families. Yuuto has an allergy to cats that cause his eyes to water and his nose to run. Both Yuuto's parents died in a car accident when he was young, making him fall into a state of depression, with his childhood friend Rinko bring in out of his depression. On his sixteenth birthday he and Rinko are walking to school when they meet an unknown girl with a katana. Later in school, a demon possess one of his classmates and tries to kill Yuuto, but the mysterious girl appears and saves him. That night he finds the girl again, but this time in his bed. She guards him from demons because a special tag, that he received from his grandparents to ward off demons, ran out of power on his sixteenth birthday.