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. Natsuo seems to care for Soubi and Ritsuka, and in turn, does nothing to harm them.
In one of the Zero-sum chapters, they are told to defeat Seimei. But because of the other's unusual power with words, the Zero team was rendered practically useless.