Mayumi Kinniku is the father of Suguru Kinniku, known as Kinnikuman, and his older brother Ataru. He serves as the fifty-seventh king of Planet Kinniku, a position that places him at the head of the heroic Kinniku clan. His most visible physical trait is the character for king written on his forehead, and he is often shown wearing a cape and shorts or a shirt with his name printed across the chest. In his youth, he was a highly accomplished wrestler, having won the Chojin Olympic championship twice and also held the title of Universal Grappling Champion. His long-standing rivalry with Harabote Muscle, the future chairman of the Chojin Olympic Committee, is a recurring thread in his history, and the two frequently engage in exhibition matches that younger audiences tend to find outdated.

Personality-wise, Mayumi is playful and often comedic, much like his younger son, but he also displays moments of poor judgment and irresponsibility. He once mistakenly abandoned Suguru on Earth after confusing him with a pig, and earlier in his parenting, he subjected Ataru to harsh training that ultimately drove the boy to run away from home. Because of that experience, he raised Suguru with far more leniency, swinging between indulgence and neglect. Despite these failings, he possesses a strong sense of justice as a righteous superhuman and genuinely cares about the safety of Earth. He often serves as a second or trainer for Kinnikuman during matches, providing advice, historical context about superhuman lore, and occasional encouragement. He is also notably henpecked by his wife, Sayuri Kinniku, who frequently scolds him for his behavior.

In the specific story of Kinnikuman New York Kikiippatsu, Mayumi appears in a supporting capacity alongside the other justice superhumans. While most of the heroes travel to New York for a tournament, he is not among those who fall to the antagonist Akuma Shogun. His role is to stand by as an observer and supporter, reinforcing the larger network of allies that Kinnikuman fights to protect. Across the broader series, Mayumi evolves from a flawed and sometimes reckless father into a wiser, more dignified monarch who ultimately passes the throne to his son.

In terms of abilities, Mayumi relies on fundamental wrestling techniques rather than flashy special moves. His signature attack is the Iron Claw, and he is known for a straightforward, ground-based fighting style that emphasizes holds and joint locks. Unlike some members of the Kinniku bloodline, he never mastered the Face Flash technique nor the three great secret techniques of the clan, partly because his body type does not build muscle easily. Nevertheless, his experience as a former champion and his extensive knowledge of superhuman history make him a valuable mentor and source of wisdom throughout the series.