Shō is a 12-year-old human boy with navy blue or black hair and brown eyes. He commonly wears a white shirt and blue trousers, and light blue pajamas at night, sometimes adding a dark blue jacket. Diagnosed at birth with rheumatic fever, he faces imminent heart surgery with low survival odds. This condition prevents him from typical childhood activities and necessitates a rest period at his maternal great-aunt Sadako Maki's home, where housemaid Haru cares for him. Largely neglected by divorced, busy parents, Shō arrives listless, passively accepting his likely mortality.

At the house, Shō catches a fleeting glimpse of Arrietty, a 14-year-old Borrower girl hidden below the floorboards. He later catches her during a nighttime borrowing mission in his room, where she drops a sugar cube. Fascinated, Shō leaves the cube and a note near her family's air vent. Despite warnings from her father, Arrietty cautiously responds. Their conversations, including one where Shō rescues her from a crow attack, forge a bond. Arrietty's courage and determination challenge Shō's resignation, inspiring him.

When Arrietty's family decides to relocate after his discovery, Shō replaces their kitchen with a dollhouse miniature to help them, inadvertently alerting Haru. After Haru captures Arrietty's mother Homily, Shō escapes his locked room, assists Arrietty in the rescue, and removes all traces of the Borrowers to protect them. During their final meeting at a creek, Shō gifts Arrietty a sugar cube, expressing how her strength gave him the will to survive his operation. She gives him her hair clip in return. One year later, Shō returns, recovered from successful surgery. Though he never sees Arrietty again, rumors of disappearing objects in nearby homes offer him hope. His name, meaning "soar, fly, glide," reflects his journey from despair to purpose.