Koji Kabuto is a Japanese teenager raised by his grandfather, Dr. Juzo Kabuto, after his parents perish in a laboratory accident. He lives with his younger brother, Shiro Kabuto, and handles domestic duties like cooking and cleaning due to Juzo's demanding secret projects. His grandfather's frequent gifts of motorcycles hone skills crucial to his future piloting. When Dr. Hell's forces murder Juzo, Koji inherits the super robot Mazinger Z, vowing vengeance and pledging to defend Japan from mechanical beasts. Initially reckless and overconfident in combat, he battles to control Mazinger Z but steadily hones strategic acumen and a deepening sense of duty. His hotheadedness sparks constant clashes with ally Sayaka Yumi, yet he fiercely protects her, risking his life when she faces peril. Their dynamic shifts from mutual antagonism to a complex bond marked by unspoken loyalty and affection. Koji also contends with rival Boss, who vies for Sayaka's attention, though they unite during pivotal battles. After defeating Dr. Hell, Koji studies UFO technology in the United States. Returning to Japan, he joins the Space Science Laboratory, designing aircraft such as the Mini Saucer and Double Spazer to aid Duke Fleed against alien invasions. His scientific aptitude echoes his family heritage, and he evolves into a photon power technology researcher. In the crossover film "Kessen! Daikaijuu," Koji pilots Great Mazinger alongside Grendizer and Getter Robo G against the oil-devouring Dragonsaurus, coordinating assaults to ensure its defeat. Facing the Mycenae Empire's invasion, he returns to pilot the heavily damaged Mazinger Z, later unearthing the formidable Mazinkaiser in a hidden lab and wielding it against renewed threats from Dr. Hell and Baron Ashura. Across his journey, Koji transforms from a brash youth into a battle-tested warrior and scientist, merging combat prowess with technological innovation. His legacy includes popularizing the mecha battle trope of vocally announcing attack names.