Aoi Sakurai is a central character and one of the heroines of the story, serving as a member of the Tokyo Central Railway Public Security Office's Fourth Vigilance Squad, also known as the "K4" unit. She is a seventeen-year-old second-year student at Sakuraka Girls' High School. Physically, Aoi has a slender frame and stands 163 centimeters tall, with crimson hair typically tied up in a bun at the back of her head and light blue eyes. While on duty, she wears the standard security uniform, which includes a turquoise tie, a skirt, and black shoes with red laces.

In terms of personality, Aoi is characterized as a tough, strong-willed, and highly assertive individual. She is known for her intense dislike of men, a trait so pronounced that her stated reason for joining the Public Security Force was to have the opportunity to shoot molesters. This attitude is reflected in her extreme catchphrase about how a caught molester should be immediately killed. She is a self-described "troublemaker" within her squad, as she constantly hopes for major incidents to occur and is always the first to rush into a crime scene, driven by a strong desire to confront criminals. Her proactive and sometimes reckless approach to law enforcement often clashes with her colleagues who prioritize passenger safety above all else. Despite this, she is a capable individual who does not fear taking risks.

Aoi's background is deeply rooted in her upbringing. Her father is a police officer serving in the Metropolitan Police Department, and it was his influence that steered her toward a life of law enforcement. From a young age, she underwent rigorous training under his guidance, including martial arts such as aikido, as well as shooting and bomb disposal techniques. This training has made her exceptionally skilled in hand-to-hand combat, with great agility, powerful kicks and punches, and proficiency in counterattacks and takedowns. She is also an expert marksman, often carrying a Beretta M92 pistol for self-defense. However, despite her tough exterior and competence, she has a fear of deep gaps, stemming from a childhood trauma of falling into the space between a train platform and a train. This fear causes her to jump decisively whenever she boards or alights from a train.

Aoi's role in the story is defined by her relationships with her fellow squad members. She develops a significant and evolving relationship with Naoto Takayama. Initially, she is cold and often hostile toward him, frequently blaming him for mistakes she herself makes. Over time, she comes to trust him and eventually develops romantic feelings for him, appreciating that he always believes in her. However, she struggles to be honest about these feelings and often resorts to threats or aggressive behavior, such as telling him not to move or she will kill him, especially when she needs his close assistance. Her jealousy is also apparent, as she once became furious enough to nearly shoot him when she saw him spending time with another female character. She also shares a close friendship with Haruka Komi, the other female member of the Fourth Vigilance Squad, whom she considers both a comrade and a rival in love. Aoi's primary motivation stems from her desire to become a public security officer, a path she pursues despite her reluctance to work directly under her father's command. She is also known by the nickname the Grim Reaper of Tokyo Station, a title earned because of her zealous willingness to shoot harassers, which has made her feared not only by criminals but also by postal workers in the area. She deeply respects a senior figure named Hitomi Gonou and is pleased to be called Gonou's successor.