A bisexual, effeminate sociopath, Rei was once saved by Masao from a near-death beating by his only friend, Yuji Aoki. However, Rei barely remembers this event as it happened in a moment of shock following Sei's death. Shortly after, Masao murders Aoki. Throughout the series, Masao is obsessed with Rei, particularly the violent side he witnessed during the rescue. He persistently tries to coax Rei into embracing this brutality and becoming like him.
Masao begins to resent Kira when he notices her efforts to nurture Rei's good side, which contradicts his own intentions. At one point, he even attempts to kill Kira to remove any obstacles in Rei's path to becoming a sociopath. Fortunately, Rei saves her just in time. When Masao realizes that Rei has made too much progress to revert to his old self, he decides to try to kill Rei. He contrives an encounter on the street where he appears distraught and wanting to change. As Rei approaches to comfort him, Masao stabs him in the stomach and flees. Rei survives and informs the police about Masao's attempt on his life.
When questioned by the police, Masao cannot recall either Rei or Kira, as if they never existed. This phenomenon, known as "repression," indicates that some part of Masao recognizes the wrongfulness of his actions and thoughts, though his conscious mind hasn't fully acknowledged it. A rare moment of humanity is seen when Rei points this out before Masao tries to stab him; Masao becomes frustrated and yells at Rei to shut up, breaking his usual calm demeanor, which often seemed unnervingly creepy in various situations.
Masao is shown to be highly manipulative throughout the series. To get close to Kira and Rei, he pretends to be innocent, pure, and naive. Later, when admitted to a psychiatric hospital, he fakes improvement and emotional recovery solely to secure his release.
