Subaru Nagahisa, a third-year student at Yukishina Junior High School, became the first child to arrive at the magical castle through the mirror, journeying from the year 1984.

Following his parents' separation and his father's remarriage, Subaru resides with his grandparents and an older brother. His brother exhibits delinquent behavior, openly dismisses Subaru as a "loser," and frequently abandons him to pursue criminal activities with friends. The grandparents, while not overtly abusive, display apathy toward both brothers, viewing them as "a lost cause" and making no effort to encourage Subaru's attendance at school. This environment inflicts profound emotional neglect, leading to Subaru's withdrawal from school and pervasive apathy.

Within the castle, Subaru presents as quiet and introspective. He reveals exceptional skill in chess, earning recognition from others as a genius player. His reserved nature begins to shift through interactions within the group, particularly influenced by Masamune's passion for video games. Inspired, Subaru develops a specific ambition to become a video game designer and actively plans for this career path during his castle visits. He surprises the other children one day by arriving with dramatically bleached-blond hair.

Subaru forms significant bonds with the other castle visitors, especially Kokoro and Masamune. These friendships provide him with a crucial sense of belonging missing from his home life. His journey involves confronting deep-seated feelings of abandonment stemming from his family situation and gradually moving towards acceptance and healing, supported by the castle's unique community.