Sou Hiyori / Midori

Sou attended the same high school as Shin Tsukimi and befriended him. They were close enough that Shin visited Sou's house regularly, though Shin was often unnerved and made uncomfortable by Sou. Shin was perceptive enough to pick up that Sou found amusement in making him nervous, but ignored it. As it turned out, Sou wasn't even enrolled in said school, so the exact reason for his presence is implied to be related to Shin.

Meanwhile, Sou had also disguised himself as a police officer and worked for law enforcement. How long is unknown, but he was present when Keiji Shinogi supposedly killed an unarmed criminal. He deliberately misled Keiji into thinking that the man was about to shoot, which led to Keiji unintentionally murdering him.[3]

At some point, while he'd been friends with Shin, Alice "killed" Sou.

Before the death game Sou had interactions with the candidates of the game posing in a variety of jobs to gain their trust and get them to sign the consent form.

The Death Game During Shin's First Trial, when Shin decided to "throw away his old self," he adopts Sou Hiyori's name.

It becomes apparent Sou is familiar to most of the Death Game participants when the majority of them find his file in Kai Satou's laptop. This includes Keiji, Q-taro, Kanna, Reko, Alice, and Shin. However, none of them could clearly remember what relation they have with him. Even Alice, who had allegedly murdered him, and Reko, who had been deeply affected by the murder case, did not remember Sou's face until they were staring right at his picture. Reko does not even mention that he was the one Alice killed, and it's clear later on that their memories were tampered with.[4]

Midori Sou with his eyes open in the coffin.

An AI doll of him is later found on the fourth floor in a coffin along with the dolls of previous candidates who did not survive the First Trial-- dubbed "Dummies." He introduces himself to the remaining six participants as the late Sou Hiyori, but

Sou and Shin met at the latter's high school and started talking. They became friends, but despite that, Shin found himself getting restless, scared, and anxious to the point of sweating if he looked into Sou's eyes. Sou was aware of the uneasiness he induced in Shin but derived enjoyment in it. In turn, Shin was aware of Sou's enjoyment in making him uncomfortable. Regardless, Shin called Sou a friend and seemingly brushed over those unhealthy aspects of their relationship.

Midori keeps a diary log of a younger Shin's AI; the version of Shin from before the latter was pulled into the Death Game, and appears possessive of him.

In Shin's own First Trial, the fact that "Shin Tsukimi" is doomed for certain death makes Shin's peaceful personality collapse and break. He then adopts a personality similar to Midori. Although, there are some noticeable differences as Shin tends to be more quiet and withdrawn.

In the route where Kanna is alive, Midori remarks to her that "since [Shin] was going to die anyways, [he] wanted to kill [Shin] with [his] own hands." When the others are reasonably shocked by this statement, he brushes it off, stating that the others don't understand his affection for Shin.

Sou/Midori is a cruelly mischievous man shrouded in mystery. He seems to have somewhat sadistic tendencies; deriving happiness from teasing and harassing others for his own amusement, even in life and death situations. He enjoys making others uneasy and playing inappropriate jokes on them.He is rather controlling; preferring to cheat or trick in order to stay in the dominant position. His energetic and playful manner of speaking is often completely contrary to the dangerous circumstan