Atsumu Matsuyuki, known by the nickname Yukiatsu, is a central character in the story of six childhood friends drifting apart after a tragedy. He is a tall, slim, and handsome sixteen-year-old with short light-brown hair and green eyes, typically seen in his prestigious high school uniform. Academically, Yukiatsu is exceptionally gifted, ranking second in his class, a testament to his hardworking and strong-willed nature.

Beneath this polished exterior, Yukiatsu is deeply tormented by the death of his childhood friend, Meiko Honma, known as Menma. He is unable to move past the event and harbors intense guilt, believing his confession of love to Menma on the day of her accident caused her to run away and drown. This unresolved grief manifests as a cold, condescending, and often hateful attitude toward his former friend Jinta Yadomi, whom he sees as a rival both for Menma's affection in the past and for her ghostly attention in the present. His animosity is fueled by a childhood inferiority complex and jealousy that Menma’s spirit appears only to Jinta, a privilege Yukiatsu desperately craves.

In an extreme expression of his obsession, Yukiatsu keeps a white sundress identical to the one Menma wore and secretly dresses in it along with a wig, wandering the forest at night to embody her presence. This shocking behavior, which he eventually stops after being discovered, highlights his manipulative and psychologically distraught state, driven by a selfish wish to see Menma again by any means necessary.

Despite his hostility, Yukiatsu finds a kindred spirit in Naruko Anjo, as they are both left behind by their unrequited feelings—hers for Jinta and his for Menma. He also shares a complex and long-standing connection with Chiriko Tsurumi, his classmate at the elite high school, who is often the only one who can see through his facades. Throughout the story, Yukiatsu's role evolves from a primary antagonist of Jinta’s quest to a reluctant participant in the Super Peace Busters' reunion. He eventually accepts Menma's ghostly presence and contributes his intelligence and resources to help grant her wish, which becomes a crucial step in his painful journey toward acknowledging his grief and beginning to heal his fractured friendships.