Junpei Onda is the younger brother of Nozomi Onda, attending Fuji Daiichi Junior High School as a member of the boys' soccer team. He possesses shaggy brown hair, brown eyes, and freckles, and by middle school had surpassed his older sister in height. Their shared childhood involved extensive soccer training under Nozomi. At Fuji Daiichi Middle School, they were teammates on the boys' soccer team coached by Kouzou Samejima.

Junpei is outspoken, often provoking Nozomi with comments that lead to physical reactions from her. He has been observed sleeping during training sessions led by Yasuaki Tani. A significant incident occurred during a newcomers' tournament match; seeking to play despite being benched, Nozomi followed Junpei into a bathroom, knocked him out, took his uniform to disguise herself and participate in the second half, leaving Junpei stripped and bound in the bathroom.

Later, Junpei attended Nozomi's high school soccer matches at Warabi Seinan High School as part of the audience, recording the games on behalf of their parents. He maintains connections with former Fuji Boys Soccer teammates like Tetsuji Yamada and Kaoru Takei. His given name "Jun" signifies obedience or submission, while "pei" conveys flatness or peace; his surname "Onda" combines "grace" or "favor" with "field" or "rice paddy."