Aoi is introduced as a second-year student at Sakuraka High School who later becomes a member of the Fourth Peacekeeper Squad. Having developed a strong dislike for men, she volunteered for the Public Security Force with the stated goal of “shooting molesters.” Her father served as a police officer with the Tokyo Metropolitan Police Department, and following his example, Aoi trained abroad in firearms handling while also studying martial arts such as aikidō. She quickly proved herself to be a gifted fighter and marksman, earning top marks during short-term training before her official assignment. Despite her respect for her father’s skills, she dislikes working under his command and strives to establish herself independently.

Unlike Naoto Takayama, who prefers his training and duties to proceed peacefully, Aoi often hopes for larger incidents to occur, viewing them as opportunities to prove herself. She always carries a pistol depicted as a Beretta 92 in the anime for self-defense, which occasionally causes friction between her and Naoto. Nevertheless, she values his unwavering trust in her abilities.

Aoi also carries emotional scars from a childhood trauma, having once fallen into the gap between a train and its platform. Because of this, she remains fearful of leaping across wide gaps when boarding or disembarking from trains. Her uncompromising approach to justice and her stated desire to target molesters earned her the nickname “Grim Reaper of Tokyo Station.” This reputation even extends to employees of the Tokyo Central Post Office, who are wary of her strictness. The only superior she openly admires and looks up to is Hitomi Gonō, whose guidance she respects.

Aoi is depicted with crimson-colored hair, usually tied into a bun at the back of her head, and light blue eyes.

While on duty with the Fourth Peacekeeper Squad, she wears the official security uniform, which consists of a turquoise jacket, skirt, tie, white gloves, black pantyhose (also worn as part of her school uniform), and black shoes with distinctive red laces. Outside of work, Aoi is seen in more casual clothing, such as dresses paired with boots, typically without additional legwear.

Aoi is portrayed as a strong-willed and hot-tempered individual, often coming across as tough and uncompromising. She initially harbors a strong dislike for men, which influences much of her early behavior, particularly her sharp and confrontational attitude. Despite her cold exterior, she is deeply committed to her duties and willing to take significant risks in dangerous situations. For example, in Rail Wars! Episode 2, she attempts to defuse a bomb herself, demonstrating her courage and determination.

Although she can be harsh, Aoi is not reckless, her decisiveness and fighting spirit reflect both her sense of justice and her desire to prove her capabilities. Over time, her interactions with Naoto Takayama and the rest of the squad reveal a more cooperative and loyal side to her character.

Naoto Takayama
Aoi first encounters Naoto before entering the Railway Academy. Initially, she treats him with hostility, often accusing him of being a pervert even when misunderstandings arose from her own actions. Over time, however, she comes to respect and trust him, eventually developing romantic feelings. Naoto later reciprocates her affection, and by the end of the light novel, the two confess their love for one another. Although Aoi departs for America for several years, they promise to wait for each other.

Haruka Kōmi
Aoi meets Haruka during the Railway Academy’s opening ceremony, and the two quickly become friends. Alongside Nana Iida, they form the female contingent of the Fourth Peacekeeper Squad. Aoi values Haruka as both a comrade and a friend, though their relationship also carries an undercurrent of rivalry due to their mutual affection for Naoto.

Aoi is an expert in hand-to-hand combat, having trained in martial arts from a young age under both professional instruction and the guidance of her father. She demonstrates exceptional agility and coordination, combining powerful strikes with precise technique. Her fighting style incorporates kicks, punches, counterattacks, takedowns, and body locks, making her effective against a variety of opponents. Her combat proficiency, alongside her skill with firearms, establishes her as one of the most capable members of the Fourth Peacekeeper Squad.