Goemon Ishikawa XIII is the thirteenth generation descendant of the legendary renegade samurai and bandit, Ishikawa Goemon. A man of profound skill and deep reserve, he is a master of multiple Japanese martial arts, including kenjutsu, battōjutsu, jujutsu, aikido, and karate, which grant him extraordinary physical capabilities. His primary and most famous weapon is the Zantetsuken, a sword of legendary sharpness capable of slicing through virtually any substance, including steel, bullets, and entire vehicles. Despite its lethal potential, Goemon prefers to use his blade in a non-lethal manner, typically cutting only an opponent's weapon or clothing, often remarking with a sense of weary ritual, "Once again, I have cut a worthless object".

His personality is defined by a quiet dignity, intense pride in his abilities, and strict adherence to a personal code of honor. Goemon is deeply introspective and spends much of his free time in meditation. He is a staunch traditionalist, dressing exclusively in a kimono and hakama, wearing his long hair in a deliberately uneven style, and refusing to eat any cuisine that is not Japanese. Failure, such as the breaking of his Zantetsuken, brings him great shame, often leading him to depart on journeys of rigorous training or meditation to atone for what he perceives as a weakness. Despite his formidable demeanor, he has notable vulnerabilities, including a naive tendency to trust seemingly innocent women, a deep discomfort around overt female sexuality, and a surprising susceptibility to scams that promise spiritual enlightenment.

Goemon's path to joining Lupin III's gang began with him as a formidable adversary. In his earliest appearances, he was tasked with eliminating Lupin to protect his clan's secrets but, after repeated failures, came to believe he could learn more by working alongside the master thief. Within the group, his relationships are complex. He shares a deep, respectful bond with the gunman Daisuke Jigen, viewing him as a fellow warrior. His connection with Lupin is more complicated; although he has developed true loyalty and often assists in serious situations, he is easily annoyed by Lupin's womanizing antics and has even sworn that his final act with Lupin will be to deliver the deathblow himself. He maintains a general distrust of the femme fatale Fujiko Mine due to her constant betrayals, though he has occasionally allied with her when their goals align. In his debut film, The Mystery of Mamo, his character adheres to these established traits, staying true to his promise that he remains with Lupin's group primarily to ensure he can one day be the one to kill him. Within this specific story, his legendary sword is broken during a battle with the antagonist, Flinch, an event that aligns with his established characterization of feeling deep shame and remorse when his abilities fail him.