Kid Muscle, whose real name is Mantaro Kinniku, is the protagonist of the story and the 59th prince of Planet Kinniku. He is the son of the legendary hero Suguru Kinniku, also known as Kinnikuman, and his wife Bibinba. Growing up during an era of peace, the young prince is initially spoiled, lazy, and quite cowardly, often preferring to play and avoid any form of training or fighting. He also has a notable lecherous streak, frequently trying to flirt with or sneak a look at girls, a trait that is somewhat toned down from the original manga for his younger age in the anime adaptation. His primary love interest is a girl named Roxanne, though he is easily distracted by many others.

Despite his selfish and fearful exterior, Kid Muscle possesses a strong, if deeply buried, heroic core. His main motivation to fight rarely comes from a sense of duty or personal glory. Instead, he consistently finds the courage to battle only when his friends are in trouble or when an opponent fights in a dishonorable way. This transformation is a key part of his character; in a crucial moment, he can tap into his hereditary power known as the Burning Inner Strength, called the Power of Ultimate Muscle in the English dub. When this power awakens, the Niku Mark appears on his forehead, and his strength, stamina, and technique increase dramatically, allowing him to overcome seemingly impossible odds.

In the story, Kid Muscle is the central figure of a new generation of justice Chojin (superhuman beings) trained at the Hercules Factory to protect Earth. He is often reluctant to participate, but his role is that of the unlikely hero who ultimately triumphs. His journey is one of stepping out of his legendary father's shadow. For much of the early story, he relies on his father's signature moves, such as the Kinniku Buster. As the series progresses and opponents learn to counter these classic techniques, he is forced to grow and innovate, eventually developing his own signature finishing moves like the Muscle Millennium and the devastating Muscle Gravity, proving he can forge his own legacy.

Key relationships heavily influence his development. His manager and trainer, Alexandria Meat, serves as his guide and conscience, teaching him about his father's great legacy which had been kept secret from him. His best friend and rival, Terry the Kid, pushes him to be better, and their friendship becomes a cornerstone of his support system. Throughout the various arcs, from the Chojin Olympics to battles against the dMp and the Demon Seed, Kid Muscle evolves from a spoiled brat who wets himself when scared into a true hero who, while still retaining some of his core personality flaws, can be counted on when it matters most. His abilities are a combination of raw power, improvisation, and a repertoire of high-impact wrestling moves, including the Kinniku Driver, the Muscle Millennium in its various forms, and submission holds like the Mantaro's Best Stretch.