Mitsuo Tanimura is a character who first appears as a lawyer that Nao Kanzaki consults after she is forced to participate in the Liar Game. He advises her to seek out Shinichi Akiyama, a former genius con artist, claiming that Akiyama is the only one who can help her win. However, this initial guise of a helpful legal professional is a deception. In reality, Tanimura is an employee of the Liar Game Tournament secretariat, and his primary function is to encourage participants to join the game and ensure the tournament progresses as planned.

Before becoming involved with the Liar Game, Tanimura was a police officer with a strong sense of justice. His past is tied to the same case that connects the protagonist Shinichi Akiyama to the game's organizers. Tanimura investigated a multi-level marketing company that had ruined Akiyama's mother, leading to her suicide. When his investigation pointed to powerful figures behind the company, his findings were ignored and he was subsequently fired from the police force. After losing his career, he was taken in by Hasegawa, a wealthy sponsor of the Liar Game, and became an employee of the secretariat. This experience fundamentally changed his worldview, leading him to adopt the cynical belief that money is the only thing that matters in the world. He even became a subordinate to Norihiko Yokoya, the man who had masterminded the very crime Tanimura had once pursued.

Tanimura's personality is complex and somewhat contradictory. He often presents a condescending and money-seeking exterior, frequently dismissing Nao Kanzaki as naive or foolish. Despite this harsh facade, he is revealed to be a tsundere character, meaning he hides a warmer, more sentimental interior behind a cold or irritable demeanor. His inner thoughts and private reactions show that he actually hopes for the games to end on a positive note and secretly supports Nao. This is because her unwavering honesty and trust in others remind him of his former self as a righteous police officer. His role in the story is that of a facilitator and observer. He manipulates Nao into continuing her participation, such as by revealing details of Akiyama's past to lure her back into a revival round, and he frequently monitors the games alongside his colleague, Eri. While he is an agent of the organization that profits from the players' misery, he is not a sadistic figure; he is a man shaped by corruption who finds himself reluctantly drawn to the very innocence he is tasked with exploiting.

Tanimura has several key relationships that define his position in the narrative. He is an employee of Hasegawa, who gave him a job after he was disgraced, and he works directly under Norihiko Yokoya, the antagonist who caused his downfall. His relationship with his coworker Eri is purely professional and somewhat cold; she is indifferent to him and they interact only as necessary for work. His most significant dynamic is with Nao Kanzaki, whom he consistently belittles yet secretly admires for the integrity he has lost. Over the course of the series, Tanimura shows subtle development. While he begins as a cynical agent of the game, his consistent observation of Nao and Akiyama begins to chip away at his hardened shell. In later games, he is seen expressing open pleasure when a competition concludes without anyone being ruined, indicating a gradual rekindling of his old sense of justice and compassion. As a former police officer and a current schemer, Tanimura does not participate in the psychological battles directly as a player. His notable ability is his skill in deception and manipulation, which he uses effectively to guide players like Nao into the tournament. He also possesses keen analytical skills, as he is often shown monitoring complex game situations and understanding the strategies employed by the players.