Kouji Tanaka is a tall, broad-shouldered man with dark hair, distinctive red eyes, and a hardened, piercing gaze. As the second Ajin officially discovered in Japan, his early experiences were defined by extreme suffering: after being captured by the government, he was subjected to years of brutal torture and lethal experimentation, which killed him over and over again and left him with a deep and simmering hatred for humanity. This resentment became the core of his personality, making him bitter, distrustful, and fiercely committed to the cause of fellow Ajin who sought to strike back against their oppressors.

He is introduced as a loyal lieutenant and second-in-command to Satō, the charismatic and manipulative leader who broke him out of captivity. Tanaka views Satō as a savior and follows him without hesitation, participating in acts of terrorism aimed at dismantling the systems that persecute Ajin. His motivation is not abstract ideology but personal vengeance: he wants to make humans suffer as he did. A capable fighter, he possesses all standard Ajin abilities, including immortality, instantaneous regeneration from any fatal wound, and the power to summon an invisible black matter entity, or ghost, that can serve as a powerful weapon and shield in combat.

The central relationship in Tanaka’s story is with Satō. Where Tanaka’s loyalty is grounded in gratitude and shared purpose, Satō ultimately betrays him, callously discarding him when it suits a larger plan. This betrayal shatters Tanaka’s sense of purpose and forces a turning point in his development. Breakbroken and disillusioned, he leaves Satō’s group and begins to chart his own course, stripped of the blind devotion that once defined him. While his role in the broader narrative is that of a secondary antagonist, his arc highlights the cycle of trauma, radicalization, and eventual reexamination of self, shifting from a man consumed by revenge to one seeking a different path after the collapse of his most meaningful alliance.