Horiguchi Hideki, known professionally as "Shark," was a boxer who raised his son Genki in poverty after his wife Minako died during childbirth. Minako’s affluent parents vehemently opposed their marriage, citing Hideki’s financial struggles and boxing career as reckless, and blamed his inability to afford proper nutrition for Minako’s death. Despite their disapproval, Hideki and Minako shared profound love, and he remained unmarried after her passing.

Hideki briefly retired from boxing following Minako’s death but later returned to the ring. This comeback culminated in a lethal bout against rising contender Kenji Seki; Hideki sustained critical injuries in the match, collapsing and dying soon after. His death left Genki orphaned, leading to adoption by Minako’s parents. The grandparents’ resentment toward Hideki intensified, faulting him for their daughter’s hardships and condemning boxing as ruinous. They highlighted his destitution and absence of material inheritance as proof of his shortcomings.

Genki cherished memories of his father as a doting, attentive parent whose affection transcended their poverty. Hideki inspired Genki to become a boxer, securing his son’s promise to follow that dream. Buried in his hometown, Hideki’s legacy fueled Genki’s ambition to face Seki while wrestling with his grandparents’ disparaging narrative of his father’s life and choices.