Ayumu Narumi is the protagonist of the story, a freshman high school student attending Sukiyomi Academy. He is a young man of average height with brown, spiky hair and an earring in his right ear. At first glance, he appears to be a stoic and somewhat carefree loner, but this demeanor masks a deeply introverted personality plagued by significant self-doubt. His entire sense of self is overshadowed by the legacy of his older brother, Kiyotaka Narumi, a genius pianist and detective whom Ayumu believes to be perfect. This comparison has led Ayumu to fear that he can never surpass his brother, to the point where he doubts his own abilities and feels he cannot accomplish anything without simply imitating Kiyotaka's actions.

Ayumu's background is far from ordinary. He was created as a clone of his brother Kiyotaka, intended by his parents to serve as a form of insurance or a test subject. Their mother, a former pianist who lost the use of her fingers in an accident, created Ayumu to be a "guinea pig" for treatments, so that if anything happened to Kiyotaka, the necessary procedures could be tested on Ayumu first. However, when Kiyotaka publicly quit his piano career and crushed his fingers, their mother deemed Ayumu useless, and he was subsequently ignored by his parents. As one of the first clones, Ayumu suffers from a very limited lifespan and numerous biological complications, a fate he shares with another character, Hizumi.

Despite his insecurities, Ayumu possesses remarkable talents. He is a world-class pianist, much like his brother, though he has largely abandoned the piano due to his feelings of inadequacy. He is also an excellent cook and, most notably, a brilliant amateur detective with top-notch reasoning abilities. His motivation to act as a detective is ignited when he is drawn into the mystery of the Blade Children, a mysterious group his brother was investigating prior to his disappearance two years earlier. Following a series of deaths at his own school linked to these children, Ayumu begins his own investigation, hoping to uncover the truth his brother left behind.

In the story, Ayumu is positioned as the supposed only hope for the Blade Children and their survival. His key relationships deeply influence his journey. He lives with and cares deeply for his sister-in-law, Madoka Narumi, who is also a police detective. He harbored feelings for Madoka before she married his brother, and some of these lingering emotions are reflected in his protective worry for her. His life is dramatically changed by his relationship with Hiyono Yuizaki, an energetic and extroverted school journalist who becomes his constant companion and support. Unlike many who doubt him, Hiyono has unwavering faith in his abilities, and she eventually becomes a major pillar of support and happiness in his life. Another significant relationship is with his brother, Kiyotaka, who is not merely a missing person but a central figure whose machinations drive the plot. Ayumu is forced to confront the truth that he is a clone and must ultimately face his brother.

Throughout the narrative, Ayumu shows notable development as he struggles against his prescribed fate. He is forced to stop merely imitating his brother and forge his own path. This development culminates in his final answer to the Blade Children: to live on, creating new opportunities and generating hope under one's own power. In the end, Ayumu is hospitalized due to his failing body. Even after losing the use of his left arm, he continues to write music, adhering to his own way of fighting destiny by living and creating for himself. His notable abilities, including his genius-level intellect, deductive reasoning, and piano proficiency, are the tools he uses to navigate the dangerous puzzles and confrontations with the Blade Children, often putting his life on the line to uncover the truth and protect others.