Aoi Futaba is a police officer assigned to the traffic division at Bokuto Station, joining the team at the start of the series. Her background is notably distinct from her colleagues, as she originally presented as male while working as a decoy in the vice unit to capture a serial rapist. During this prolonged undercover operation, a profound personal transformation took place, leading her to continue living as a woman after the assignment concluded. This experience is central to her character, and she expresses her identity with the clear statement that her body is male but her heart is female.

In terms of personality, Aoi is characterized by an exceptionally feminine demeanor and appearance, often described as surpassing even the female officers in these qualities. She possesses a gentle and refined nature, and she excels in traditional domestic arts such as flower arrangement and cooking, skills that impress even the most capable of her peers. Despite her typically docile and soft-spoken manner, she is far from defenseless. Her training and past experience allow her to demonstrate surprising combat prowess in high-pressure situations, reminding those around her that she is a capable officer.

Her role at Bokuto Station is primarily as a member of the traffic section, though her unique skills are not often utilized in this capacity. She forms a close partnership with fellow officer Yoriko Nikaido, quickly becoming fast friends with her and the rest of the station after any initial confusion about her gender identity is resolved. Her motivations are largely driven by a desire to live authentically and perform her duties as a police officer. Her romantic life becomes a recurring theme, as it is established that she is attracted to men. This leads to several storylines, including a notable incident where she rejects a marriage proposal from a famous actor who was unaware of her history, which marks a rare occasion where she is seen in male clothing. Another storyline involves her anxiety over meeting a man she has fallen in love with online, fearing his reaction to learning she is a transgender woman.

Before joining the police force, Aoi was an accomplished basketball player, known for her strategic mind and ability to make crucial shots, a past that earned her many female admirers. In the original manga source material, her athletic background is depicted as golf rather than basketball. Physically, her appearance features a subtle but notable change across the anime's seasons, with her hair color shifting from brown in the first series to black in the second. Her journey of self-discovery is deeply connected to her former superior, Udamura Kumanosuke, who was involved in the sting operation that led to her transition. The support she receives from her coworkers, who treat her as any other woman, provides a stable and affirming environment for her development throughout the series.