Tonpei stands among the three primary child characters, retaining his original Japanese name and recognized as the onion boy. He radiates high energy and athleticism, his hot-tempered nature tempered by an underlying kindness and eagerness to assist others. As the informal leader of the "Kooky Kids," he drives their adventures and problem-solving missions across the village and surrounding forest. His family resides on a farm, anchoring him in the community's agricultural setting.

Throughout the series, Tonpei consistently engages in the group's escapades, collaborating with Peako and Kuruko to address villagers' challenges and confront local antagonists. A key narrative saw him actively support Kuruko when her grandfather initially forbade her from attending school, showcasing his loyalty and commitment to friends. His development emerged clearly during a critical forest fire, where he helped rescue the trapped Kuruko, escaping the danger zone alongside Peako.

The series concludes with Tonpei solidifying his bond with the other children, collectively pledging to maintain their friendship indefinitely. His spirited and protective nature consistently surfaces in collaborative efforts to ensure community safety and resolve interpersonal conflicts among villagers.