Kenji Seki is a professional boxer whose match against Hideki Horiguchi culminates in Horiguchi's fatal injuries, an unintended outcome Seki later regrets. This incident positions Seki as an indirect catalyst for the protagonist's journey. Afterward, Seki ascends to become an undefeated world champion, dominating boxing while his success starkly contrasts with the struggles of those linked to the Horiguchi family.

Beyond the ring, Seki harbors personal feelings for Yuko Ashigawa, igniting a rivalry with Eiji Mishima, who shares this affection. The animosity between Seki and Mishima intensifies when Seki discovers Ashigawa's enduring loyalty to Mishima despite his own advances. Consumed by jealousy, Seki physically assaults Mishima.

Renowned in boxing as "Syklon Seki," his formidable reputation and championship status establish him as the protagonist's long-term goal for a decisive ring victory. The narrative resolution diverges between adaptations: the anime concludes with a direct confrontation between protagonist and Seki, while the manga omits this encounter.