A bisexual effeminate sociopath who was often bullied by his only friend, Yuji Aoki. Rei had saved Masao from being beaten to death at one point in time, but barely remembers this event as the action was impulsively taken when Rei was still in shock over Sei's death. Soon after this incident, Masao kills Aoki. Masao is rather obsessed with Rei throughout the series, but to be more specific he is obsessed with the brutal side he saw when Rei saved him. Throughout the series Masao tries to convince Rei to let that brutal side of him take over and to become just like him. He strongly starts to dislike Kira after he sees that she is trying to nurture the good side in him, which is the exact opposite of what he's trying to do.
Masao is proven to be extremely manipulative throughout the series. To get close to Kira and Rei he tricks them by acting very innocent, pure, and naive. And when later on in the series he gets admitted into a psychiatric hospital, he completely fakes getting better and feeling emotions just so he can get out.