Koyuki Matsubara is Ryo Takagi's girlfriend and fellow student. She maintains their romantic relationship but frequently reprimands Ryo physically for impulsive actions, like using his motorcycle to whip up her skirt. Her father operates a clinic that treats Ryo's severe injuries from street fights and training. Koyuki demonstrates traditional domestic traits, cooking for Ryo, including one experimental seafood cake with squid and shrimp that caused him to gag.

She supports Ryo's transition from street fighting to organized boxing, encouraging him alongside friends Satou "Sorry" and Suu. Koyuki balances affection with assertiveness, coyly refusing Ryo's persistent advances to sleep together while remaining emotionally invested in his well-being. Her presence represents a stabilizing influence, contrasting his volatile "Burning Blood" persona.

Koyuki witnesses Ryo's defeat by Jin Moriyama, the event catalyzing his entry into boxing. She later observes his rigorous training under coach Otobe, including unconventional methods like swimming during typhoons. Though concerned for his safety, she endorses his pursuit of a rematch with Moriyama in the boxing ring.