Jijii, whose real name is never revealed, operates as the central mastermind behind the violent upheaval that drives the story forward. His nickname, meaning "old man" or "geezer," is deceptive, as his aged and withered appearance is a fabrication. He is actually in his early thirties but has used extensive plastic surgery to give himself the face of an elderly man, while his body remains extremely muscular and powerful due to heavy steroid abuse. This duality between his frail-looking face and his extraordinarily strong physique reflects the hidden and manipulative nature of his character.

As the handler and creator of the killer Ichi, Jijii is defined by a cold and calculating personality. He is a master manipulator who orchestrates a complex scheme to pit rival Yakuza clans against each other, aiming to destabilize the criminal underworld of Shinjuku. His motivations extend beyond simple financial gain, such as stealing a large sum of money from the Anjo gang. He is driven by a deeper, arguably ideological goal to act as a vigilante, balancing the scales of good and evil by pitting lesser evils against greater ones, effectively cleansing the streets by provoking a self-destructive war within the Yakuza.

Jijii's primary tool for achieving his goals is Ichi, a psychologically broken young man whom he has molded into a lethal assassin. He achieves this by implanting false memories in Ichi's mind, the most significant being the fabricated trauma of witnessing a classmate named Tachibana being raped in high school. This invented memory conflates sexual arousal with violence and shame, giving Jijii a psychological trigger he can use to activate Ichi's homicidal state at will. Jijii's role is not that of a direct combatant but a puppet master, feeding misinformation to the sadomasochistic enforcer Kakihara, using a cleaning crew to erase evidence, and carefully directing Ichi to murder key figures.

He is surrounded by a small band of associates, including the drug-addicted Inoue and the firearms expert Noboru, who assist in his elaborate plans. His key relationship is with Kakihara, the story's primary antagonist, whom Jijii methodically manipulates from the shadows, leading him on a destructive path that results in the downfall of his entire gang. This psychological chess game forms the core of the narrative, with Jijii representing cold, calculated intelligence opposing Kakihara's chaotic, pain-driven obsession.

Throughout the story, Jijii shows little to no direct development or change in his personality, remaining a steadfast and hidden orchestrator until the very end. However, his physical capabilities, hidden behind his elderly facade, are notable. Despite his short stature, his steroid-enhanced physique grants him superhuman strength, allowing him to easily snap the neck of a much larger man, Takayama, when confronted. This shocking revelation underscores the danger he poses even without his pawns.

After achieving his goal of destroying the Anjo gang and engineering the deaths of both Kakihara and, indirectly, many of his own pawns, Jijii's fate is ambiguous. He is ultimately found hanged from a tree. The circumstances of his death remain unclear; it is left to interpretation whether this is a suicide driven by the completion of his nihilistic mission or a murder, possibly an act of revenge by Takeshi, the young boy whose father was killed in the crossfire of Jijii's war. This final, silent image of his hanging body serves as a grim and unresolved conclusion for the master manipulator.