Kiri Haimura is the main protagonist of the series, a middle school student who carries an unusual and powerful secret. He is a seemingly ordinary boy whose most defining personal trait is an intense, almost obsessive fascination with cutting hair. This habit has made him an oddity in his neighborhood since childhood, though it is a compulsion that runs far deeper than a simple hobby. This obsession leads him to the secluded Iwai Mushanokōji, the Hair Queen, whose magically uncuttable hair he discovers he can finally sever using his cherished pair of scissors.

Kiri is generally presented as a pleasant, agreeable, and well-mannered young man, which allows him to gain people's trust relatively easily. However, this amiable exterior belies a much darker and more complex inner world. As the wielder of a Killing Good, he is the current Author and descendant of Norma Grayland, an infamous historical serial killer to whom the scissors originally belonged. The scissors themselves are known as The Severing Crime Edge, a cursed artifact with a grim lineage. Upon learning that Iwai is the target of a deadly game where other Authors seek to kill her to have a wish granted, Kiri’s primary motivation crystallizes into a dedicated and unwavering pledge to protect her at all costs. This vow of protection is not merely a sense of duty; it becomes the core of his existence, as his emotional and psychological stability becomes directly tied to her safety.

The central relationship of Kiri’s life is with Iwai Mushanokōji, who serves as his Instead. In the world of Killing Goods, an Instead is a person on whom an Author uses their weapon in a non-lethal, ritualistic way to temporarily satisfy the weapon’s bloodlust and stave off the homicidal impulses that would otherwise consume them. For Kiri, the daily act of cutting Iwai’s hair purges the violent urges inherited with the Crime Edge. This necessity forges an incredibly deep, symbiotic bond between them that quickly develops into a romantic relationship. Iwai is not just his charge but the very anchor of his humanity; should she die, Kiri would lose his Instead and inevitably succumb to the murderous desires of his lineage, turning into a true killer. His interactions with other characters are largely defined by this mission, as he clashes with various other Authors who come to claim Iwai’s life, viewing them as threats to both the girl he loves and his own salvation.

Kiri’s character undergoes significant development as he grapples with the grim legacy of his ancestral weapon. Initially presented as a boy with a simple, if strange, fixation, he is forced to confront the true, horrific nature of his inheritance. At first, the scissors’ primary power is their supernatural sharpness, capable of severing flesh and bone as easily as hair. As the story progresses and his Author blood awakens, he unlocks the weapon’s deeper, more sinister ability: pain augmentation. Any wound inflicted by the Crime Edge feels exponentially worse to the victim, where a small cut can hallucinatorily feel as though an entire limb has been severed, a power derived from his ancestor Grayland’s enjoyment of causing immense suffering. This realization forces Kiri to confront the inescapable weight of his heritage. Ultimately, his journey leads him to use the Crime Edge to execute the malevolent spirit Zewulfa, an act that marks his first and final kill, allowing the curse to be lifted and the scissors to be put away as a mere memento. This final act demonstrates his refusal to be defined by his ancestor’s bloody history, choosing instead to use the tool for a final, necessary act of closure.