Atsumu Matsuyuki, universally known as Yukiatsu, stood as a core member of the childhood friend group, the Super Peace Busters. Throughout elementary school, he exuded confidence and leadership aspirations, frequently positioning himself as a rival to Jinta Yadomi for the group's lead. Yet, he consistently deferred to Jinta, accepting his place within their dynamic, however reluctantly. Unspoken romantic feelings for Meiko "Menma" Honma complicated his relationships; he never confessed these feelings while she lived. This hidden affection fueled underlying tension, especially concerning Jinta's closeness with Menma, though Yukiatsu generally maintained positive bonds with other members like Naruko Anjo (Anaru) and Chiriko Tsurumi (Tsuruko).

Five years after Menma's accidental death, Yukiatsu projected the image of a popular, academically successful high school student, excelling in studies and athletics. This facade concealed profound psychological trauma rooted in guilt over her death. He harbored a belief that his attempt to confess his feelings on the day of her accident contributed to the tragedy. His unresolved grief manifested in increasingly obsessive and harmful behaviors. Resentment towards Jinta deepened, driving Yukiatsu to antagonize and ridicule his former friend constantly. In an extreme act fueled by jealousy upon learning Jinta claimed to see Menma's ghost, Yukiatsu donned a white sundress to impersonate her. This deception aimed to manipulate the group into believing Menma had returned, discrediting Jinta's experiences and exposing Yukiatsu's unhealthy inability to process his loss.

Yukiatsu's coping mechanisms proved largely self-destructive and isolating. He privately engaged in the ritual of wearing Menma's dress while reminiscing about her, his heavy breaths betraying deep emotional turmoil. His initial refusal to accept the possibility of Menma's ghost stemmed directly from his guilt and jealousy. Despite his antagonism, he formed a tentative bond with Anaru, recognizing their shared experience of unrequited love—hers for Jinta, his for Menma—which allowed them moments of mutual confiding.

A significant shift unfolded as the group gradually reunited to help fulfill Menma's final wish. Yukiatsu eventually moved past his denial and acknowledged her ghostly presence. Collaborating with the others, he participated in efforts like organizing a fireworks display to help her pass on. When this plan failed, a heated group confrontation ensued. During this emotional breakdown, Yukiatsu, alongside everyone else, openly confessed his vulnerabilities and regrets. This shared catharsis proved crucial for group reconciliation and ultimately enabled Menma to move on. A pivotal step in his healing arrived with a handwritten letter Menma had left for him before her departure. Reading it allowed him to release years of pent-up anguish and guilt.

In adulthood, Yukiatsu's life trajectory reflected lingering struggles with his past. He initially pursued a banking career in Tokyo but abandoned it to return to Chichibu, signaling ongoing difficulty moving forward. He attempted a romantic relationship with Chiriko Tsurumi, who had long held feelings for him. This relationship, however, ultimately ended, evolving into a stable and enduring friendship. This outcome underscored his journey towards accepting that some profound emotional connections might not resolve as he once hoped, signifying a complex form of closure.