Junpei Onda is a middle school student and male soccer player with shaggy brown hair, brown eyes, and freckles. He is the younger brother of Nozomi Onda. Junpei played for the Fuji Boys Soccer Team during middle school alongside teammates Tetsuji Yamada and Kaoru Takei under coach Kōzō Samejima; he was sometimes observed sleeping during training sessions.

Frequent conflicts define his relationship with Nozomi. Junpei's tendency to speak his mind often provokes her, escalating to physical fights. A significant incident occurred during a middle school match: while Junpei was in the bathroom, a benched Nozomi forcibly took his uniform, tied him up, and left him there. She then impersonated him to join the game, highlighting Junpei's role in catalyzing her actions and his vulnerability within their dynamic.

Later, during the events of the main series sequel, Junpei attended the Warabi vs. Urawa high school match as an audience member with former teammates Yamada, Takei, and Yasuaki Tani. Instructed by his parents, he recorded the match for Nozomi, acting as her cameraman and showing his continued, passive involvement in her soccer pursuits.

Junpei's personality blends a supportive familial role with bluntness. He performs tasks for his sister's benefit, like recording her games, yet his candid remarks frequently trigger her aggressive reactions, reinforcing their contentious but interconnected relationship. No substantial character development or changes in his role beyond these events are documented.