Boy #13 Yuichiro Takiguchi, an otaku, is very naïve and a bit too friendly. He hides with his friend Tadakatsu.
In the novel and manga, when Mitsuko attempts to kill Tadakatsu, they capture her. Yuichiro sympathizes with Mitsuko, and intervenes when Tadakatsu tries to kill her. Tadakatsu accidentally shoots Yuichiro, and then Mitsuko kills Tadakatsu. In the novel, Mitsuko kills and kisses Yuichiro, while in manga, Mitsuko rapes an injured Yuichiro before killing him.(Source: Wikipedia)