Seito Hojoin stands as the primary rival to protagonist Shunichi Domoto, entering the series as reigning Japan Yoyo Champion. His first yoyo, a gift from his father, gained profound emotional weight when his mother passed away while holding it. This tragedy forged a complex dynamic with his father, who pressured Seito to abandon yoyoing and become a master pianist. Their conflict peaks when Seito abandons an imminent victory at a major pianist competition to defend his yoyo title at the Japan Champion Carnival.

Seito first encounters Shunichi after witnessing Shunichi defeat Benkei Musashimaru. Offended by Shunichi’s dismissive attitude toward competitive yoyoing, Seito crushes him in their initial match. This defeat ignites Shunichi’s resolve to challenge Seito at the national tournament. As defending champion during the Japan Champion Carnival, Seito advances relentlessly through the brackets, emerging as Shunichi’s ultimate obstacle. He reaches the semi-finals against Benkei Musashimaru, where he acknowledges Benkei’s effort and sportsmanship despite the latter’s limitations.

Seito wields the Hyper Dragon, a prototype yoyo developed by D-Project. Combined with his exceptional technical skill and experience, this equipment establishes him as a formidable force. His presence continuously pressures Shunichi to rapidly develop new techniques. The narrative anchors their rivalry as the central catalyst driving both characters’ motivations within the tournament’s framework.