Rion Mizumori, a first-year junior high student in Hawaii, enters a magical castle through his bedroom mirror. Attending boarding school after his parents sent him away, he carries the weight of his older sister Mio's death from a terminal illness seven years prior. His mother's quiet resentment over his health deepens his emotional isolation.

Though not enrolled at Yukishina Junior High—Mio's former school—Rion yearns to attend. Hawaii's time zone allows him to access the castle during his local afternoon, participating after his regular classes. Initially reserved within the group, his identity as a soccer player surfaces alongside his profound grief and sense of dislocation.

His character evolves through shared trauma with the six other children. A pivotal revelation uncovers the castle's overseer, the Wolf Queen, as his deceased sister Mio. In the climax, Mio removes her wolf mask to bid him farewell, granting Rion closure.

The story reveals the seven children hail from the same universe but distinct timelines spanning over 40 years. Rion exists in the 2005 timeline alongside Kokoro Anzai. After the castle fulfills its purpose and their memories fade, Rion reunites with Kokoro in their shared timeline, marking his reintegration into a supportive connection.