Atsumu Matsuyuki, nicknamed Yukiatsu, belonged to the childhood friend group known as the Super Peace Busters alongside Jinta Yadomi, Meiko Honma, Naruko Anjo, Chiriko Tsurumi, and Tetsudo Hisakawa. In elementary school, he harbored romantic feelings for Meiko "Menma" Honma, viewing Jinta as a rival who consistently surpassed him in various activities and seemingly earned Menma's affection more easily. Following Menma's accidental death, Yukiatsu outwardly projected a facade of normalcy, excelling academically and athletically as a model student. Internally, he grappled with profound guilt over her death, intense self-loathing, and unresolved jealousy toward Jinta. His grief spiraled into disturbing behaviors, including wearing Menma's dress and a wig to impersonate her ghost near their childhood hideout—an act deliberately aimed at undermining Jinta's claims of seeing her spirit, driven by his own inability to perceive her and a lingering obsession. Yukiatsu initially treated Jinta with hostility and condescension, frequently attempting to discredit his efforts to reunite the group for Menma's final wish. His relationship with Chiriko "Tsuruko" Tsurumi remained complex; she harbored long-standing romantic feelings for him and often acted to mitigate his actions and shield the group from his damaging behaviors, acknowledging his underlying pain. Through reuniting the Super Peace Busters and confronting their shared trauma, Yukiatsu gradually acknowledged his guilt and jealousy. He participated in fulfilling Menma's wish, leading to a painful reconciliation with Jinta and forging a deeper, complicated bond with Tsuruko. Ten years later, Yukiatsu worked at the Chichibu town hall after a period as an elite banker. His significant relationship with Tsuruko persisted, though they had not married due to what she described as a lack of "the right timing," leaving their status ambiguous. Learning their childhood hideout, the Secret Base, faced demolition during a reunion barbecue, Yukiatsu impulsively declared he would buy the land to preserve it. He retracted this after Tsuruko highlighted its impracticality, demonstrating his acceptance that some changes are inevitable despite valuing their shared history. During the gathering, he reflected on the carving "The Super Peace Busters will always be friends" inside the base, silently reaffirming the enduring spirit of their bond even as their lives evolved. His adult interactions revealed a perspective balancing fierce nostalgia with an understanding that moving forward honors their past and their promise to Menma.