Atsumu Matsuyuki, nicknamed Yukiatsu, stands 180-181 centimeters tall and weighs around 68 kilograms. His light brown hair features parted bangs, complementing his green eyes. He typically wears a white collared school shirt bearing a breast-pocket emblem, dark blue jeans held by a brown leather belt, a burgundy tie, and brown shoes, often accompanied by a brown leather-strapped wristwatch. As a child, he sported a blue-and-yellow cap with a fish emblem, a white collared shirt, denim shorts, white mid-calf socks, and orange-and-white running shoes. Yukiatsu belonged to the Super Peace Busters childhood group alongside Jinta "Jintan" Yadomi, Naruko "Anaru" Anjou, Chiriko "Tsuruko" Tsurumi, Tetsudō "Poppo" Hisakawa, and Meiko "Menma" Honma. His unrequited feelings for Menma, who favored Jinta, culminated in a confession on the day of her accidental death. He internalized blame for her demise, profoundly shaping his later behavior. Following Menma’s death, Yukiatsu adopted a condescending and antagonistic demeanor, particularly toward Jinta. He excelled academically, attending an elite high school with Tsuruko while Jinta failed to enroll. Despite outward confidence, unresolved trauma drove obsessive actions: he kept a white sundress mirroring Menma’s in his closet, later cross-dressing in it with a wig to wander forests at night impersonating her. This deception aimed to manipulate friends into believing Menma’s ghost returned, fueled by jealousy over Jinta’s claims. Confronted, he admitted seeking connection and coping with guilt. Yukiatsu confided in Anaru, bonding over their shared unrequited love—hers for Jinta, his for Menma—and acknowledged their kinship as "kindred spirits." His vulnerability contrasted with manipulative tendencies, provoking group conflicts and publicly ridiculing Jinta. He openly challenged efforts to fulfill Menma’s wish, declaring, "Let’s stop this nonsense about granting Menma’s wish," and accused Jinta of perpetuating collective pain. A pivotal shift occurred when Yukiatsu accepted Menma’s spiritual presence and collaborated to help her pass on, participating in initiatives like launching fireworks for her wish. In humility, he begged on his hands and knees for assistance. After initial failures, a heated group argument sparked mutual vulnerability confessions and collective catharsis. Menma’s posthumous letter, praising his diligence and strength, released his lingering anguish. Post-Menma’s peaceful passing, Yukiatsu reconciled with friends and grew closer to Tsuruko, hinting at a budding romantic relationship. This evolution marked his departure from hostility and emotional isolation toward integration and peace.