Kirika Yumura is a central figure in the story, a young woman whose very existence is shrouded in mystery, beginning with her own mind. She is introduced as a teenage girl living in Japan who has woken up with near-total retrograde amnesia, unable to recall anything about her past except for a single word: Noir. Her real name, her parents, her place of origin, and even the details of her life up to that point are never explained, leaving her identity as a profound enigma that she is desperate to solve. Her initial memories consist only of the contents of a deserted house: a forged student ID card with the name Kirika Yumura, a Beretta M1934 pistol, and a pocket watch that plays a haunting melody.

Physically, Kirika has a deceptively frail and unassuming appearance. She is a small-framed teenager with short black hair cut in a chin-length bob and large reddish-brown eyes. Her usual outfit of a white jacket, light green shirt, blue skirt, and distinctive ruby-colored shoes reinforces her image as an ordinary, somewhat conservative high school student, which stands in stark contrast to her true nature. This contrast is central to her personality; her soft-spoken, quiet, and sometimes naive demeanor belies her identity as an exceptionally gifted and instinctive killer. Her personality is often described as enigmatic and emotionally distant, but this is not a sign of true coldness. Rather, she struggles to understand and express her feelings, and her lack of visible emotion in the face of death is a source of deep personal distress.

Kirika's primary motivation is the search for her lost past, which she believes holds the key to her identity. She initiates this pilgrimage by contacting Mireille Bouquet, a professional assassin in Paris, using the melody from her pocket watch as a lure. To Mireille, this melody is the only clue to the murder of her parents, making Kirika an object of both suspicion and necessity. Together, they form a partnership under the codename Noir, a legendary title for two maidens who govern death, and they work as assassins while seeking the truth. Kirika’s role in the story is to be one half of this legendary duo, serving as a catalyst that pulls Mireille out of her solitary life and forces them both to confront a sprawling conspiracy involving a powerful secret society known as Les Soldats.

Despite her youth, Kirika possesses the skills of a superbly trained killer, the result of being chosen by Les Soldats at a very young age to be a candidate for Noir. Her abilities are a matter of trained instinct rather than conscious memory; her body remembers what her mind has forgotten. She is an expert marksman, but her true lethality lies in her incredible creativity and ability to adapt. She can turn practically any object into a deadly weapon, from a student ID card to a fork or the axle of a toy car. She also demonstrates exceptional tactical wit, using the environment to her advantage, such as scattering debris on a floor to hear approaching enemies or using coins to create diversions. Her fighting style is a brutal and efficient form of Waif-Fu, using her small stature and an opponent's momentum against them.

The key relationship in Kirika's life is with her partner, Mireille. Their alliance begins in tension, with Mireille only agreeing to help Kirika on the condition that she will eventually kill her. Over the course of their journey, this uneasy partnership evolves into a profound, if unspoken, bond. Another significant figure is Chloe, a rival assassin who idolizes Kirika for her exceptional killing skills and believes they are destined to become Noir together, creating a complex and tragic triangle. Kirika’s past is also directly tied to Mireille's trauma; it is revealed that as a very young child, around the age of five or eight, she was the one who carried out the assassination of Mireille's parents on the orders of Les Soldats. The dying wish of Mireille's mother, Odette Bouquet, was for Kirika to take care of her daughter, a request that haunts Kirika and ultimately helps shape her final decisions.

Kirika undergoes significant internal development throughout the series. Initially an emotionless girl who mechanically performs her deadly tasks, she slowly begins to confront the weight of her actions. Her first moments of genuine happiness are seen in simple pleasures, like finding a stray cat. The greatest change comes when she is forced to kill Chloe, an act that finally allows her to understand the profound sadness of taking a life. It is at this point, experiencing deep grief for the first time, that she begins to truly feel human. In her final confrontation with Altena, the leader of the Soldats, she rejects the role of a "True Noir" who would continue to kill as an instrument of fate, choosing instead to try and atone for her sins. This represents her ultimate rejection of the identity crafted for her in favor of forging her own path, one defined not by her past as a killer, but by the bond she has formed with Mireille.