Following her parents' deaths, Aoi Mishina inherits one of three special "Musumet" helmets alongside her sisters, Kurenai and Midori. Their father, Dr. Mishina, and Dr. Kishida developed these helmets to harness power from shooting stars and protect world peace. Aoi and her sisters continue this mission through the secret organization "MET".

Wearing her helmet transforms Aoi into Musume Blue, altering only her headgear, gloves, and boots while leaving her regular clothes intact. Consequently, she often rushes into missions still wearing whatever outfit she had on when alerted—be it her school uniform, athletic wear, swimsuit, or other situational attire.

As a civilian, Aoi is a blue-haired, blue-eyed teenage student, frequently seen in her school uniform. She ranks among her school's top athletes and demonstrates sharp intelligence, especially for strategic decisions. Though her personality can show harsh or self-centered moments, she consistently supports others as Musume Blue. She occasionally assisted her father in his office as a child.

Her relationships with her sisters involve typical sibling squabbles, yet they fiercely protect each other. She initially treats Yamato Shirai as an assistant but develops romantic feelings for him after recognizing his dependability. She shows little affection toward her uncle Hiroshi Kushida, though retains some trust. She interacts formally with MET members Murasaki and Nako (who maintains her blue helmet). Mutual animosity defines her relationship with the Saotome sisters, though they reluctantly cooperate when Yamato faces danger.

Aoi juggles her superhero responsibilities with typical teenage concerns like school and romance. However, the relentless emergency demands from MET severely restrict her opportunities for personal relationships or leisure activities.