Kozue Matsumoto attends the same school as Baki Hanma and lives next door to him, her mother acting as his landlady. Her father died fighting in the Underground Arena, which informs her initial unease regarding Baki's violent combat path. She harbors deep feelings for Baki but denies them when questioned, especially early on. Disillusioned by his underground fighting life, Kozue repeatedly urges Baki to quit, fearing for his safety. Visiting the grave of Emi Akezawa, Baki's mother, shifts Kozue's understanding of his resolve, though she continues to disapprove of the brutality in his pursuits. After Baki's first major tournament match, he openly confesses his feelings, solidifying their bond. Their relationship progresses to intimacy just before Baki's critical fight against Ryuukou Yanagi, an event described as enabling Baki to become a "full man". During the Maximum Tournament, Kozue witnesses Baki's matches firsthand after his invitation to the Tokyo Dome's underground arena. She struggles to comprehend the fighters' willingness to endure severe injuries, leading to a heartfelt conversation where Baki explains their dreams of becoming the strongest. Her presence includes moments of vulnerability, like tearfully embracing Baki out of concern upon hearing threats against his life. Before Baki's final match against Jack Hammer, they share their first kiss in a corridor. Kozue later pleads with Baki to stop fighting during his brutal bout with Jack, though he continues. Kozue demonstrates unexpected assertiveness defending her relationship. She confronts Kaoru Hanayama after he suggests Baki seek sexual partners elsewhere to avoid distraction, scolding him for disrespecting their bond and lightly kicking his leg. She similarly intervenes during Baki and Yujiro Hanma's climactic fight, chiding them for prolonging their battle through conversational pauses. Physically, Kozue has brown eyes and hair. She initially wears it in braids, later adopting a ponytail or wearing it down, with subtle style changes including her bangs. She stands approximately 160 cm tall and dresses modestly, often in her school uniform—a dark ensemble earlier, transitioning to a white-shirt version. In an anime-exclusive event, school thugs kidnap Kozue, prompting her friend Mai to alert authorities and Baki to intervene. While briefly desired by Muhammad Ali Jr., Kozue reaffirms her commitment to Baki, explaining she views Ali Jr. like a child needing consolation after his loss. Yujiro Hanma later explicitly approves of her as Baki's partner, crediting her role in strengthening his son. During Baki's recovery from poison administered by Ryuukou Yanagi, Kozue's tears interact with the toxin, aiding his cure. She continues supporting Baki in later conflicts, walking him to school after his battle with Pickle and reminiscing about past events like his fight with Ali Jr. Her courage surfaces again when she attempts to physically assist Baki against Yanagi, unaware of the threat's severity. Kozue possesses no formal combat abilities and initially struggles to comprehend the martial arts world. However, her tolerance for its violence increases after witnessing numerous fights during the Maximum Tournament and other events. Her character arc centers on balancing her love for Baki with her aversion to his violent path, ultimately becoming an unwavering source of emotional support.