Aoi Aoi is a former member of the first generation of the Phantom Triggers, serving as a shooter within the CIRS division at Mihama Academy before the institution was reorganized into SORD. Her full name is noted as unusual, but she personally liked it because she found it easy to remember.

In terms of background, Aoi Aoi is an upper-class assassin from a facility known as Hollow House, which was designed to train children as killers. She belongs to a generation from before regulations were tightened, making her one of the most dangerous operatives from that era. Her role was less that of a national defense asset and more akin to a tool of national offense, a so-called violence device rather than a defense device. She was given as a gift to her master, Sengoku Ichiru, when Ichiru left her family home.

Aoi Aoi possesses a distinct and fierce personality. She is described as loud, short-tempered, and rebellious, with a love for money. Her speech pattern is rough and direct, often using informal language. She is known for her smoking habit and her tendency to speak her mind without pretense. Her actions are guided by her own set of rules, and she lives freely without concern for general common sense, a trait that is seen as part of her charm. This straightforward nature means she rarely has a hidden agenda. Despite her viciousness, there is a noted purity to her character, suggesting a complex individual whose harsh exterior belies a more fundamental honesty.

Her motivations and role in the story are intrinsically tied to the protagonist, Aoi Haruto. She is his master, responsible for looking after him when he was a baby and entrusted to her by Sengoku Ichiru. The annual trip to visit her grave serves as a central plot point for one volume of the story, during which the surviving first-generation members reminisce about her life and the events that led to her death. The narrative explores how Haruto, who was raised and educated by her, deeply mourns her loss, an event that humanizes him and reveals his emotional depth.

Key relationships define her legacy. Her relationship with her master, Sengoku Ichiru, is complex; she was given as a gift and often tried to resist his orders. This resistance against her conditioning caused her great agony, described as feeling like her brain was burning. However, her most significant relationship is with the young Haruto. Despite her dangerous and seemingly irresponsible methods, which included taking a baby into combat zones, on a motorcycle, or even using him as a distraction against an enemy, she was his primary caretaker and teacher. She was responsible for teaching him hand-to-hand combat and survival skills, shaping him into the person he becomes. Her approach to training him was unorthodox; she would pummel him in fights but encouraged him to think for himself rather than blindly follow her instructions.

Aoi Aoi's development is presented primarily through flashbacks after her death. She is remembered as a young woman who was both extremely violent and yet possessed a strange purity, indicating a character who lived by a brutal but consistent personal code. Her story serves as a cautionary tale about what happens when a person is turned into a living weapon, and her final moments are a subject of reflection for those she left behind.

Her notable abilities include proficiency as a shooter with a handgun and expertise in unarmed fighting. As an assassin from the pre-regulation era of Hollow House, she is considered exceptionally dangerous and highly skilled in combat. She is also known to use a sword in battle, and her tactics are pragmatic to the point of being considered ruthless, as she will use any available resource, no matter how unconventional, to defeat her enemy.