Akari Mamiya is a first-year student at Tokyo Butei High School, a specialized institution where students known as Butei are authorized to carry weapons and resolve incidents. She belongs to the Assault Department, a discipline focused on direct combat, but her official rank is E, which places her near the bottom of the school's evaluation system. Despite her low standing, Akari harbors an intense admiration for the S-rank Butei second-year student, Aria H. Kanzaki, and her primary goal is to become Aria's Amica, a formal junior partner and apprentice. In a surprising turn, she manages to pass Aria's notoriously difficult entrance test, officially becoming her Amica and setting the stage for her growth.

In her daily life, Akari often comes across as clumsy, carefree, and somewhat naive, a demeanor that belies a remarkably stubborn and resilient core. She is deeply loyal and protective of her friends, sometimes to a fault, and possesses a natural charisma that tends to attract and unite other female students around her, even those who initially act as rivals. This ability to form strong bonds contrasts sharply with her mentor Aria, who is more of a solitary operative. Akari's motivation is largely driven by her devotion to Aria, and she works tirelessly to live up to the example her senpai sets, even though her methods are often different, relying on cooperation rather than solo action. This devotion also manifests as a strong, somewhat childish jealousy towards Kinji Tohyama, a male Butei from the main series who is Aria's partner, as Akari views him as a rival for her mentor's attention and actively tries to keep them apart.

Akari's role in the story is that of a protagonist whose journey is about uncovering her own hidden potential. The cheerful and somewhat inept exterior she presents is a deliberate choice, stemming from a traumatic past. She is a descendant of the Mamiya clan, a family of assassins descended from the secret police of the shogunate. Two years prior to the story, the criminal organization IU attacked her family, seeking to steal their secret assassination techniques. The attack resulted in the destruction of her home, the scattering of her clan, and the poisoning of her younger sister, Nonoka, with whom she now lives. This trauma has given her a deep-seated reluctance to engage in lethal combat, leading her to suppress her true abilities and adopt a foolish persona to avoid conflict. Her desire to become a Butei is partly a way to honor her mother's teachings while trying to find a new, non-lethal purpose for her skills.

The narrative follows Akari as she gradually confronts her past. A key part of her development is learning to reconcile her family's lethal legacy with the rules of being a Butei, which forbid killing. Her true talent is an incredibly precise form of marksmanship, a killing technique called Tobiu-gachi, where she can achieve one hundred percent accuracy by aiming for lethal areas like the heart and head. However, the Butei law against killing forces her to aim for limbs to disarm opponents, which she is not trained for, resulting in her deceptively poor aim. Over the course of her training and battles, she begins to accept her heritage, learning to use her family's techniques in non-lethal ways or when the situation is dire enough to justify their use. Her rank gradually improves from E to D and eventually C as she overcomes various challenges and confronts enemies from her past, such as the IU agent Kyochikutou.

Her key relationships form the core of her character arc. Her bond with Aria is the most significant, evolving from one-sided worship into a genuine partnership of mutual respect. She has a close, though sometimes strained, friendship with her classmate Shino Sasaki, who is fiercely possessive and protective of Akari. Her relationship with her sickly younger sister, Nonoka, is a primary source of motivation and fear, as Akari desperately wants to protect her from any further harm. Her attitude towards Kinji Tohyama is one of constant, low-level antagonism, as she schemes with other characters to interrupt any private moments between him and Aria.

In terms of notable abilities, beyond her suppressed lethal marksmanship, Akari's primary weapon of choice is a Micro Uzi submachine gun, which she uses alongside a tactical knife. Her poor performance at range is partly due to her choice of weapon, as the Micro Uzi is known for poor accuracy, which ironically helps her avoid accidental lethal shots. Her true strength lies in her family's assassination techniques, including a powerful move called Takamakuri. As she develops, she learns to harness these abilities, ultimately proving herself to be a formidable fighter with a unique skill set that balances immense lethal potential with a strong personal code against killing.