Aoi Matsubara is a 15-year-old high school student specializing in the mixed martial art "Extreme Rules." She possesses short, dark blue hair with spiky bangs, light blue eyes, a slim build, and stands 153 cm tall. Her typical attire includes a sailor school uniform, athletic wear, or boxing gloves during training sessions.

Originally a karate practitioner, she shifted focus to establishing Extreme Rules as a recognized combat style. This decision drew rejection from traditional martial artists, particularly Sakashita-senpai, who dismissed the discipline as illegitimate. Aoi idolizes fellow practitioner Ayaka Kurusugawa, viewing her as the benchmark for skill. Despite rigorous training, she initially displayed timidity in competitive matches, struggling with confidence.

Her development centers on resolving conflicts through martial prowess. In *Remember My Memories*, she defeats Ayaka Kurusugawa in a tournament with a decisive roundhouse kick. This victory validates Extreme Rules' effectiveness while reconciling her rift with Sakashita-senpai, whom Ayaka had previously defeated. The win reinforces her determination and reduces hesitancy in high-stakes situations.

Aoi exhibits a hotblooded, energetic, and hardworking personality, channeling intense effort into daily training. She maintains social obliviousness ("donkan") but engages in friendly rivalries like that with Yoshie, who openly mocks Extreme Rules as trivial. Her interactions occasionally imply unspoken romantic interest in Hiroyuki Fujita, though this remains secondary to her martial arts pursuits.