Shin, formally known as Arata Kishimo, is a 17-year-old high school student and Mio Miyamasu's childhood friend. Following his father's death, he lives apart from his mother. Shin possesses a supernatural legacy: astral projection abilities inherited through the Kishimo bloodline.

This power activates critically after Mio suffers a traffic accident and is drawn into the metaphysical World Border realm. Visiting her comatose body in the hospital, Shin receives a mysterious call from Mio herself. His soul instantly leaves his physical form, projecting him into World Border. There, he joins Mio, her pet fish transformed into the humanoid Kiku-chan, and an alien guide named Gimon.

Within World Border, Shin actively works to help Mio return to the human realm. Their journey involves seeking aid from entities like the goddess Mrs. Mori and confronting guardians such as Mogari. When Mogari obstructs Mio's path after she secures a train ticket for her return, Shin teams up with Gimon and Kiku-chan to ensure she reaches the departure point. Although Mio initially fails to reunite with her body upon returning to the human world, Shin carries her unconscious form to a cliff holding significance in their shared memories. His presence and actions there trigger Mio's recollection of her suppressed feelings for him. This emotional awakening provides her essential "desire," enabling her full return to her body.

Throughout this crisis, Shin exhibits protective instincts and deep emotional investment in Mio's safety, particularly visible in his unwavering drive to bridge the realms. His involvement in the supernatural events following her accident is fueled by their relationship, marked by mutual affection and unresolved tensions.