Dazai Osamu was an ability user who could Nullify others abilities. He was once the "Demon Prodigy" in the Port Mafia, where he committed hundreds of crimes, then alongside his partner, Chuuya Nakahara, who could Manipulate Gravity with his ability. They were the deadliest pair, Soukoku, or Double Black. After the death of his loy friend, Odasaku, he decided to change sides for the sake of Odasaku, and joined the Armed Detective Agency to solve crimes instead.