Horiguchi Genki, raised solely by his father Hideki Horiguchi following his mother's death in childbirth, inherits a legacy shaped by boxing. Hideki, a former professional turned itinerant fighter, battled poverty and malnutrition while training, inspiring Genki's early dream of becoming a champion. Their close bond included shared runs for training. Genki witnessed his father collapse and die from heart failure after a match against national champion Kenji "Cyclon" Seki, blaming Seki and vowing to defeat him in the ring. Adopted by grandparents opposed to boxing for safety reasons, Genki secretly sustained his training and goal.

As a high school amateur boxer, his journey featured rigorous training and encounters with pivotal figures: a teacher evoking his mother's memory, high school champion Mishima, and coach Eikichi Nagano, who refined his skills. Transitioning from clandestine practice to formal coaching, Genki evolved from a determined novice to a disciplined fighter, converting grief into drive.

Physically small yet energetic, his name—"Go for it, Energy!"—mirrors his spirited nature. Personality blends fierce determination with compassion, marked by loyalty to friends and mentors. Key relationships include supportive best friend Hiroshi, who offers emotional balance; love interest Natsu, a childhood friend pursuing her own ambitions; and a complex dynamic with Seki, later revealed to harbor guilt over Hideki's death.

Genki's arc centers on reconciling vengeance with athletic growth. Acknowledging Seki's remorse shifts his motivation from defeating Seki to honoring his father through boxing excellence, advancing from amateur bouts toward professional aspirations.