Gyro Zeppeli, whose full name is Julius Caesar Zeppeli, is a central figure from the Steel Ball Run race, hailing from the Kingdom of Naples, Italy. He is approximately 24 years old and comes from a unique family lineage that serves the Neapolitan monarchy as both royal executioners and medical doctors, a combination justified by the need for an intimate understanding of human anatomy. His personal motivation for entering the grueling cross-continental race is not for the prize money or glory, but to save the life of a young shoe shine boy named Marco. Marco was sentenced to death for failing to report a conspiracy, and as the appointed executioner, Gyro defied his family's traditions and the law. To secure the boy's amnesty, Gyro seeks to win the race as a special envoy of the king, hoping to use his victory as leverage for a pardon.

Despite his grim profession, Gyro possesses a flamboyant, cocky, and often eccentric personality, which stands in stark contrast to his more serious partner, Johnny Joestar. He is known for his distinctive laugh, "Nyoho!," and his gold teeth engraved with "GO! GO! ZEPPELI". While he can be rude and mocking to rivals, he demonstrates a strong personal code and a compassionate side, particularly toward children and the innocent. His altruistic nature is a source of conflict with his stern father, Gregorio Zeppeli, who disapproves of such sentimentality. Gyro also has a quirky fondness for teddy bears, carrying one with him during the race, and enjoys telling bad puns and singing self-composed songs about cheese to Johnny. He holds a superstitious belief that the goddess of victory rides with him, which is why he refuses to let another woman ride his horse.

In the story, Gyro initially acts as an unwilling mentor to Johnny Joestar, a paraplegic former jockey who becomes fascinated by Gyro's mysterious technique. Over the course of the race, their tactical alliance deepens into a genuine and profound friendship as they face numerous enemies and search for the scattered parts of a holy corpse. Gyro's role evolves from a competitor focused solely on winning to a dedicated teacher and partner. A key moment in his development occurs during his battle with the stand user Ringo Roadagain, where he stops racing solely for Marco and begins to internalize the race as a personal journey of self-discovery. This growth is essential, as Johnny notes that Gyro initially lacked the selfish hunger of a true champion because he inherited his skills rather than earning them through suffering.

Gyro's notable abilities center on the Spin, a hereditary technique that utilizes specially crafted steel balls. By throwing these balls with a perfect rotation, Gyro can create a variety of powerful effects, including manipulating an opponent's muscles and nerves, wringing water from objects, and even canceling pain. After obtaining the Right Eye of the Saint's Corpse, he temporarily gains an ability named Scan, which grants him enhanced vision and the power to see through objects and identify weak points. Gyros ultimate technique is the Golden Spin, a perfect rotation achieved by synchronizing his steel ball with the natural galloping of his horse, Valkyrie, along the lines of the Golden Rectangle. This mastery allows him to manifest his stand, Ball Breaker, a small, humanoid-like entity that can bypass dimensional barriers. Its primary ability, senescence, causes any living thing it touches to age rapidly, a power potent enough to penetrate the defenses of Funny Valentine's Love Train.