Atsumu Matsuyuki, known as Yukiatsu in childhood, was part of a tight friend group called the Super Peace Busters with Jinta "Jintan" Yadomi, Naruko "Anaru" Anjou, Chiriko "Tsuruko" Tsurumi, Tetsudō "Poppo" Hisakawa, and Meiko "Menma" Honma. As a child, he sported light brown hair often hidden beneath a unique blue and yellow cap marked with a circular fish emblem. His typical outfit included a white collared shirt, very short denim shorts, white mid-calf socks, and orange-and-white running shoes.

Within the group, Atsumu nurtured a competitive streak and a simmering inferiority complex toward perceived leader Jinta. He resented Jinta's natural authority and the attention he commanded, especially from Menma. This jealousy fueled manipulative attempts to undermine Jinta's standing among their friends. Despite this friction, the group shared a strong bond anchored in their secret base and shared adventures.

A pivotal childhood moment unfolded when Atsumu confessed romantic feelings to Menma. This declaration came moments after Jinta angrily rejected Menma's question about becoming his bride. Reeling from Jinta's rejection, Menma fled the scene upon hearing Atsumu's admission. Tragically, she slipped near a river and drowned while retrieving a discarded letter meant for Jinta. Atsumu carried profound guilt, convinced his ill-timed confession directly caused her fatal accident.

Following Menma's death, Atsumu retrieved a hairpin she had rejected earlier that day and secretly kept it. This hidden token became a secret emblem of his unresolved grief and enduring attachment. His emotional turmoil during this period also shaped his later, complicated relationship with Chiriko, who found and preserved the discarded hairpin, recognizing the depth of his unhealed pain. The shared trauma of Menma's loss fractured the Super Peace Busters, scattering the friends for years.