Hei, also known as BK-201 and nicknamed the Black Reaper, is the masked and highly skilled protagonist of Darker than Black: Kuro no Keiyakusha. He works as a Contracting Agent and assassin while secretly searching for his missing sister, Bai. Calm, analytical, and deadly in combat, he often uses electric wires and tactical planning to overpower enemies. Throughout the series, Hei serves as the central figure around whom the mystery of the Contractors, the Syndicate, and the changing world events unfold.