Shiro Kabuto is the younger brother of Koji Kabuto. He first appears in the original Mazinger Z series as a grade school-aged boy, primarily serving in a supporting role that often provides comic relief. His physical appearance varies: in the main TV series, he sports brown hair and black eyes, typically wearing a green short-sleeved shirt with red outlines, jean shorts, and brown shoes. The Mazinkaiser OVA design aligns more closely with the original manga, featuring short black hair, a yellow shirt, dark blue shorts, and white sneakers.

Personality-wise, he embodies friendliness, playfulness, and fierce loyalty towards his older brother, often worrying about Koji's safety during battles. Despite his youth, he displays wisdom beyond his years, coupled with a hot temper, impulsiveness, and a tendency towards accidents due to overeagerness or bad luck. He proves hardheaded, sometimes resorting to physical confrontations with bullies rather than diplomatic solutions.

Family relationships drive his development. After the presumed deaths of his parents and the loss of his grandfather Juzo Kabuto, he becomes deeply reliant on Koji as his primary guardian. Shiro experiences significant grief over Juzo's death and initially resents his father Kenzo Kabuto for hiding his survival for years. He eventually reconciles with Kenzo after learning the reasons for the secrecy, forming a bond by the end of Great Mazinger. He also develops a close friendship with Kenzo's adopted son, Tetsuya Tsurugi.

His interpersonal connections extend beyond family. He initially views Boss and his gang as bullies but later joins them as a friend in Great Mazinger, participating in their comedic misadventures. At school, he befriends Lorelai Heinrich, forming a sincere connection that ends tragically when she is revealed as an android controlling a Mechanical Beast, leading to her destruction by Koji. This event deeply distresses Shiro. Later, in Great Mazinger, he grows closer to his classmate Haruka Shiratori, eventually referring to her as his girlfriend.

Shiro's role evolves across installments. In Mazinger Z, he mainly provides comic relief and emotional support for Koji, facing moments of panic during critical battles like the Mycenae Empire's attacks. In Great Mazinger, his character gains prominence: he pilots his own robot, Robot Junior (later Ichinana), demonstrating competence against Warrior Beasts despite limited experience. This series highlights his personal growth, including integration into Boss's gang and his evolving relationship with Haruka.

In the Mazinkaiser OVA, he retains his loyalty to Koji but exhibits a more mischievous and occasionally lecherous side around female characters like Sayaka Yumi or the twins Lori and Loru. His role here remains largely supportive, assisting in non-combat scenarios such as identifying impostors during attacks.

The Shin Mazinger reboot reimagines him with enhanced technical aptitude, portraying him as a skilled inventor and engineer assisting in constructing and upgrading Mazinger Z. This version intensifies his tragic connection to Lorelai, framing it as a robosexual relationship ending in heartbreak. He pilots the Aphrodite A robot, showcasing heightened responsibility and combat involvement.

His abilities include hand-to-hand combat skills taught by Koji, enabling him to fend off groups of bullies or brainwashed adversaries. He is also adept at baseball, incorporating this into his piloting tactics with Robot Junior. Despite his age, he proves resilient in battle, holding his own against formidable enemies.