He is the fighter of the male Zero pair. He is one of Nagisa's experiments, and presumably the second generation of Zeros. He first makes an appearance in volume 2, when he and Youji corner Soubi when he's alone. When Soubi wins the battle, Natsuo tells him that he and Youji have no place to return to if they lose a battle. Soubi offers to let them live at his apartment, and Natsuo warily accepts.

(Source: Wikipedia)