Narutaki Soutaro is a boy of 16 with spiked hair and green eyes. He is a Dark Doctor, a person capable of exterminating and curing Disease Demons that still roam around. He is rather short for his age, which lead to many people believing he is a child. Though he is somewhat of a coward, once he starts something, he will definitely finish it. His counterpart, Izumo Momo is his childhood friend and nurse who usually summons the medical weapons used to get rid of the Byouma. He likes books, and is a money spender, often picking Momo's pocket to buy old books. He is currently looking for the legendary book "Kaitai Shinsho Zero".
It is revealed later that he doesn't have Dark Doctor's blood on him so he cannot touch the Doctor's equipments directly without wearing a special glove. He is actually came from an assassin's clan