Jijii, whose real name is never revealed, is a central figure in the story who operates under an alias that means Old Geezer. Despite his withered and aged appearance, he is actually a man in his early thirties. His elderly look is a deliberate disguise, the result of extensive plastic surgery performed to make him appear older and unrecognizable. Beneath this frail exterior, however, he possesses a heavily muscled and powerful physique, which he maintains through the abuse of steroids.

Jijii is a master manipulator whose primary role is the hidden puppet master orchestrating the story's violent conflicts. His personality is defined by a cold, calculating, and machiavellian intelligence. He is a man driven by a grand, destructive ambition, aiming to dismantle powerful yakuza organizations by turning them against one another. His methods are indirect and psychological; he prefers to work from the shadows, using others as his instruments rather than engaging in direct confrontation.

His primary motivation and darkest deed is the creation of the killer Ichi. Jijii has psychologically conditioned Ichi, implanting false memories of childhood trauma, including the rape of a classmate named Tachibana. This fabricated past conflates violence with sexual arousal in Ichi's mind, turning him into a weapon that Jijii can direct at his targets. Through this manipulation, Jijii uses Ichi to assassinate Yoshio Anjo, the head of the Anjo yakuza family, and steal the gang's money, an act designed to ignite a gang war.

Jijii's key relationships are all defined by control and exploitation. He acts as a handler to Ichi, providing him with direction and approval. He also employs other outcasts and criminals to further his schemes. His primary antagonist is Kakihara, the sadomasochistic enforcer of the Anjo family. Jijii feeds Kakihara false information, manipulating him into torturing and killing rival gang members, thereby escalating the very conflict he set in motion. Theories within the narrative suggest a more personal motivation, positing that Jijii himself was the victim of the traumatic events he planted in Ichi's mind, making his war on the yakuza a form of elaborate, vicarious revenge potentially aimed at Kakihara.

Throughout the story, Jijii undergoes minimal outward development, remaining a consistent and enigmatic force. However, his ultimate fate provides a resolution to his role. After his machinations lead to the final, ambiguous confrontation between Ichi and Kakihara, Jijii is found dead, hanged from a tree. The nature of his death is deliberately ambiguous, leaving it uncertain whether it was a suicide—perhaps from despair or a sense of completed purpose—or a murder, potentially an act of retaliation from the son of one of Kakihara's men.

Jijii possesses several notable abilities. His primary skill is psychological manipulation, allowing him to brainwash and control individuals like Ichi. Despite his short stature, his steroid abuse has granted him a deceptive and explosive level of superhuman strength, enabling him to perform feats such as snapping the neck of a much larger man with ease. He is also resourceful and highly intelligent, able to devise complex, multi-layered plans that span the entire criminal underworld. While he is most often seen in his frail, old-man guise, the revelation of his true, muscular form serves as his most startling and notable display of power.