Kotaro Higashi is a wandering young man returning to Japan with aspirations to become a professional boxer. His journey begins when he plants Tigris Flower bulbs, which unexpectedly mutate into the monster Astromons. During an attack by the monster Oil Drinker, Kotaro intervenes using construction equipment to restrain it, showcasing his bravery and earning recognition from the defense team ZAT. After Astromons critically injures him, the Ultra Brothers revive him, merging him with Ultraman Taro through the intervention of Mother of Ultra, who appears as a human woman in green. She gifts him the Ultra Badge, enabling his transformation into Ultraman Taro. As a ZAT member, Kotaro balances his duties fighting monsters with his role as a surrogate older brother to Kenichi and Saori Shiratori, children of a sea captain who offered him shelter. His personality combines impulsiveness with a strong sense of justice, often demonstrated through protective instincts toward civilians and children. During missions, he confronts monsters like Birdon, which temporarily kills both him and Zoffy before their resurrection, and Tyrant, a composite monster that defeats multiple Ultra Brothers before Taro's victory. These battles highlight his growth from reliance on others to a more strategic fighter. The relationship between Kotaro and Taro involves shared consciousness and memories, with Kotaro retaining his human identity while accessing Taro's powers. The Ultra Badge serves as both a transformation device and a symbol of their bond. Depictions vary regarding their origins: some present Kotaro as a human merged with an existing Ultra entity, while others imply he was reborn as Taro himself. Ambiguity also surrounds Mother of Ultra's role, as she is suggested to have taken the form of Kotaro's deceased mother. In the series finale, Kotaro voluntarily returns the Ultra Badge to Mother of Ultra, renouncing his transformation ability to prove humanity's self-reliance. This follows the death of Captain Shiratori, which fuels his determination to defeat the alien invader Valky without Ultra powers. Using environmental tactics and ZAT weaponry, he lures Valky into an oil refinery and ignites an explosion, eliminating the threat through human ingenuity alone. After this victory, he leaves ZAT to travel the world, symbolizing his completed journey toward self-sufficiency. In the alternate continuity of the ULTRAMAN manga, Kotaro is reimagined as a Japanese photographer in America. After investigating an alien robbery, he is fatally wounded but resurrected with superhuman abilities via extraterrestrial drugs. Adopting the heroic identity "Taro," he crafts a handmade suit and allies with the SSSP. His powers include a volatile Flame State that engulfs his body in fire, requiring a specialized suit from Yapool to control the energy. This version operates independently, fighting threats like the Star of Darkness and confronting societal tensions between humans and aliens. Post-series lore indicates Kotaro eventually resumes interstellar activities as Taro after his period of travel, later becoming an instructor for the Inter-Galactic Defense Force. His legacy includes mentoring newer Ultras like Mebius and fathering Ultraman Taiga, though his human form does not reappear in subsequent narratives.