Aoi Matsubara is a 15-year-old high school student born on January 19. She stands 153 cm tall with a petite, slim build, short dark blue hair with spiky bangs, and light blue eyes. Her typical attire includes a sailor school uniform, athletic wear, and boxing gloves during training. Originally a karate practitioner, she shifted her focus to establishing the mixed martial art known as Extreme Rules as a recognized combat style. This decision drew rejection from traditional martial artists, particularly her rival Yoshie Sakashita, who dismissed the discipline as illegitimate. Aoi idolizes fellow practitioner Ayaka Kurusugawa, viewing her as the benchmark for skill. Despite rigorous training, she initially struggles with confidence in competitive matches. In the story Remember My Memories, she defeats Ayaka Kurusugawa in a tournament with a decisive roundhouse kick, validating Extreme Rules' effectiveness and reconciling her rift with Sakashita. Aoi is hotblooded, energetic, and hardworking, channeling intense effort into daily training while maintaining a social obliviousness. Her friendships with classmates Hiroyuki Fujita and Kotone Himekawa bolster her spirit, and she campaigns to found a combat sports club despite peer skepticism and institutional resistance.