Koji Tanaka is the second Ajin, or demi-human, officially discovered in Japan. Shortly after his identity was revealed, he was captured by the government and subjected to years of brutal experimentation in a secret facility, where he was killed and revived hundreds of times as scientists tested the limits of his immortality. This prolonged torment left him with a deep, burning resentment toward humanity and the authorities that allowed his suffering. He was eventually rescued by the terrorist leader Sato, who had seen footage of the experiments and infiltrated the facility to free him and other captive Ajin. Following his rescue, Tanaka became Sato’s loyal lieutenant and a core member of the Ajin resistance, embracing a violent, us-against-them worldview.

Tanaka is a tall, broad-shouldered man with sharp, piercing eyes, and he carries himself with a quiet, almost impassive demeanor. He does not easily show frustration or anger in most situations, but he is utterly unflinching in the face of bloodshed and willing to use extreme force against anyone he perceives as an enemy. His captivity fundamentally shaped him: he views all humans as oppressors and sees Sato’s vision of an Ajin-dominated world as the only path to safety and justice. He operates as a front-line soldier in Sato’s operations, relying on his immortality and his invisible black ghost, or IBM, to overwhelm opponents. He is methodical in combat and shows no hesitation when carrying out orders.

In the story, Tanaka serves as a secondary antagonist who executes some of Sato’s most aggressive plans. He is involved in the raid on a hospital to kidnap Kei Nagai’s sister, hoping to draw Kei into the conflict. He clashes directly with other characters such as Izumi Shimomura and Kei himself, whom he despises for refusing to join the fight against humanity and instead cooperating with human authorities. Tanaka’s relationship with Sato is one of absolute loyalty and gratitude; he sees Sato as his savior and follows him without question. This loyalty, however, is eventually betrayed: Sato uses Tanaka as a pawn and ultimately abandons him, forcing Tanaka to separate from the group and fend for himself. This betrayal marks a turning point in Tanaka’s development, stripping him of the guiding purpose he had found under Sato.

As an Ajin, Tanaka possesses full regenerative immortality, allowing him to recover from any fatal wound within seconds. He can also summon a black ghost, a powerful invisible entity that he uses for combat, though his control over it and his strategic thinking are less refined than those of more experienced Ajin like Sato. His abilities make him a formidable fighter, but his trauma and rigid worldview often limit his capacity for independent strategy or negotiation. Over time, his experiences in captivity and his subsequent role in Sato’s rebellion define him as a character driven by pain, vengeance, and a need for belonging, one whose journey reflects the central themes of persecution and moral ambiguity in the series.