Atsumu Matsuyuki, known by his childhood nickname "Yukiatsu," was a founding member of the Super Peace Busters friend group. As a child, he possessed light brown hair and green eyes. His typical outfit featured a blue and yellow cap bearing a circular fish emblem, a white collared button-up shirt, very short denim shorts, white mid-calf socks, and orange-and-white running shoes. Yukiatsu demonstrated intelligence and diligence from an early age, traits Menma later praised in a letter acknowledging his hardworking and strong nature. However, he harbored a fierce inferiority complex toward the group's charismatic leader, Jinta "Jintan" Yadomi. Yukiatsu perceived Jintan as surpassing him in games, leadership, and crucially, in winning Menma's affection. This jealousy fueled competitiveness and resentment, intensified by Yukiatsu's own deep feelings for Menma. These unrequited feelings reached a climax on the day of Menma's accident. Collaborating with Naruko "Anaru" Anjou, who held feelings for Jintan, Yukiatsu instigated a confrontation where Anaru directly asked Jintan if he loved Menma, aiming to create distance between them. The plan backfired when a flustered Jintan called Menma "ugly" and fled. Menma pursued him, resulting in her fatal fall into the river. Earlier that same day, Yukiatsu had confessed his feelings to Menma and given her a hairpin as a token. Menma rejected him, returning the hairpin. Yukiatsu believed his confession directly caused Menma's death, as she fled after rejecting him. He carried profound guilt, feeling responsible for her accident and stating, "If only I hadn't said those words to Menma, then she wouldn't have died." Witnessing Menma's death or its immediate aftermath alongside Tetsudō "Poppo" Hisakawa deepened his trauma. This guilt, intertwined with unresolved grief and persistent jealousy toward Jintan, fueled his adolescent psychological struggles, including cross-dressing as Menma and antagonistic behavior. His complex relationship with Chiriko "Tsuruko" Tsurumi also originated in childhood. Tsuruko, aware of Yukiatsu's feelings for Menma, retrieved and secretly kept the rejected hairpin he had given Menma, symbolizing her own unrequited affection for him.