Earth Masamune, often referred to as Earth Masamune due to his given name, is a central character in the story. He is a boy who arrives at the mysterious castle as the fifth participant, joining the group of children summoned by the Wolf Queen. Masamune is a second-year student at Yukishina Junior High School, and his original time period is the early 2010s. Physically, he is a teenage boy with brown hair cut short to his ears and matching brown eyes, and he is typically seen wearing glasses.

His most defining characteristic is his avid passion for video games. This interest is not a mere hobby but a core part of his identity, shaping his analytical and logical approach to the world around him. When the group faces the puzzling revelation that they attend the same school yet cannot find evidence of each other's existence in their own daily lives, it is Masamune who proposes a theory of parallel universes to explain the discrepancy. This highlights his tendency to use frameworks from his favorite medium to understand complex real-world problems. His enthusiasm for gaming also has a profound impact on one of his companions; his deep dedication inspires another castle visitor, Subaru Nagahisa, to pursue a career as a video game designer.

Within the group dynamic, Masamune serves as a thoughtful and sometimes hesitant voice of reason. While he shares in the camaraderie and the search for the hidden key, he also demonstrates a desire for self-improvement and mutual support. At one point, he musters his courage to suggest that the group help each other face their personal struggles in the real world, even proposing that they try returning to school together. This proposal, which the group agrees to act on by a specific date, shows his development from a boy content in the castle's refuge to one willing to take proactive steps toward change in his own life. His journey is marked by this growing willingness to translate the strength found in the group's friendship into action outside the mirror's portal.