Yukio exists as a half-demon, isolated and rejected by both human society and the demon world due to his hybrid nature. This dual rejection enforces a life of solitude. His core motivation drives him on a persistent quest to find his biological mother, defining his personal journey.

During this search, Yukio discovers a village living under the terror of a monster called the Long-Neck, which demands yearly sacrifices of young maidens. Observing that one intended victim, Shizue, resembles his foster mother, Yukio chooses to intervene. This decision temporarily halts his quest, revealing his capacity for empathy and protective instincts despite his typical detachment from human affairs. His actions demonstrate a willingness to confront threats when personal connections, even symbolic ones, are evoked.

After confronting the Long-Neck and rescuing Shizue, Yukio resumes his original journey. A narrative voice underscores his ongoing struggle, expressing a cautious hope for future improvement. This reflects his continued existence as an outsider, moving through a world that largely scorns him, driven by the unresolved goal of discovering his origins.