Kozue Matsumoto is Baki Hanma's schoolmate and girlfriend. Her mother, Kinuyo Matsumoto, serves as Baki's landlady, with both families living as neighbors. Her father fought and died in the Underground Arena. She appears as a pretty Japanese teenager with brown eyes and brown hair, standing about 160 cm (5'3") tall. She typically wears her hair braided or in a ponytail and dresses in ordinary clothing, often her school uniform. Her hairstyle changes slightly during the Maximum Tournament saga, uncovering her forehead. Initially quiet and shy, Kozue cares deeply for Baki but struggles to reconcile with his violent life as an underground fighter. She repeatedly urges him to quit, disillusioned by the brutality. Her understanding deepens after visiting the gravesite of Baki's mother, Emi Akezawa, though she continues to disapprove of his choices. Despite her reserved nature, she asserts herself defending her relationship, confronting Kaoru Hanayama for suggesting Baki seek sexual experiences outside their commitment. Her character development includes showing significant courage during Baki's battles. She supports him emotionally before his fight with Ryuukou Yanagi; their physical relationship prior to that confrontation marks a pivotal moment for Baki's maturity. During Baki's poisoning by Yanagi, her tears interact with the toxins to aid his recovery, earning unexpected approval from Yuujirou Hanma. Yuujirou calls her "an excellent woman" and remarks Baki "got the best woman in the world". In the Maximum Tournament saga, Kozue discovers Baki's secret life when he invites her to the Tokyo Dome's underground arena. Witnessing the violent world firsthand, she questions the fighters' motivations but gradually develops greater tolerance. Key moments include her hallway encounter with Jack Hammer and her emotional plea for Baki to stop fighting during his match against Jack. Baki openly expresses his affection here, sharing their first kiss in the arena corridor. Later, during the Most Evil Death Row Convicts saga, she and Baki share their first formal date. Unaware of the danger, she remains unharmed when Spec attempts an attack, which Baki anticipates and Hanayama intercepts. Her relationship faces a brief challenge when Mohammad Alai Jr. pursues her affections; she rejects him, choosing Baki and describing Alai Jr. as childlike. She later reaffirms her commitment while walking Baki to school after his fight with Pickle. In the final battle between Baki and Yuujirou, Kozue intervenes to scold them for prolonging their fight with conversational pauses. Yuujirou acknowledges her positively during this encounter. While the series suggests their emotional bond changes over time, potentially influenced by external factors like Alai Jr.'s interference, she remains a consistent emotional anchor for Baki. Kozue possesses no combat abilities and initially struggles to comprehend the martial arts world. Her resilience grows through repeated exposure to high-stakes fights, including the Raitai tournament, though she remains a non-combatant throughout.