Inoue is a minor but pivotal character in Ichi the Killer, serving as a low-level operative within the organization of the character known as the Old Geezer, or Jii-san. His background is rooted in organized crime; he is a former member of the Anjo yakuza family who was excommunicated before the events of the story. At the time of his appearance, he works alongside the Old Geezer and his crew, though his personal history places him on the fringes of the yakuza world he once belonged to.

In terms of personality and behavior, Inoue is depicted as a man deeply compromised by addiction. He is a heroin user, a habit that defines his immediate circumstances and makes him vulnerable. His dependence on the drug is severe, and he suffers from withdrawal symptoms when unable to satisfy his craving, a state which leaves him frantic, desperate, and prone to erratic behavior. This addiction is a key vulnerability, marking him as the weak link within his group and the most easily exploitable source of information for their enemies.

Inoue's primary role in the story is to serve as the instrument through which the central mystery of the narrative is unraveled. After the disappearance of the Anjo family's leader, the sadistic enforcer Kakihara begins a brutal investigation to find the killer. Kakihara deduces that Inoue is a heroin addict and uses his dealer to track him down. The confrontation that follows is the defining moment for the character. Kakihara finds Inoue in a state of drug-addled hysteria, having torn apart his hideout in a paranoid frenzy. Kakihara then subjects him to a method of torture, playfully pushing long pins into his body in a game reminiscent of a children's toy. Under this extreme duress and facing death, Inoue breaks down completely. He reveals the crucial information that Anjo is dead and names the killer: Ichi. Furthermore, in a final, shocking revelation that speaks to his disturbed nature, he tells Kakihara that he is next on Ichi's list.

His key relationships are defined by his criminal associations. He is a subordinate of the Old Geezer, though his loyalty is ultimately superseded by his personal desperation during his torture. The most significant relationship in his final moments is with his tormentor, Kakihara. This interaction is not a relationship of equals but one of predator and prey, where Inoue becomes the source of critical intelligence.

Inoue demonstrates no notable combat or physical abilities; his importance derives entirely from the information he possesses. His death is as notable as his life, occurring at the hands of Kakihara after he reveals the truth about Ichi. In a final display of the story's transgressive themes, Inoue makes one last request of his killer. Having confessed that he himself has sexually violated the corpses of his own victims, he asks Kakihara to perform the same act on his body after he dies, expressing a desire to understand the experience from the other side. Kakihara, moved to tears by the sheer audacity of the request, agrees. This moment concludes Inoue's arc, transforming him from a desperate addict into a figure who, even in death, articulates one of the core, disturbing fixations of the world he inhabits.