Youta lives on Shio Island with his younger sister, Umi. He meets the protagonist soon after she arrives and helps her integrate into the island community. His persistent invitations for her to swim and jump from a bridge into the sea with other children form a key part of his supportive role. Early on, however, he shows no awareness of her recent loss; he casually mentions local fathers involved in traditional activities, unknowingly highlighting his initial lack of understanding about her grief.

He acts as a bridge connecting the protagonist to island life, encouraging her participation in communal events. He attends the tōrō nagashi ceremony, a Buddhist tradition of floating lanterns for ancestors, reflecting his deep roots in local culture. When an emergency arises involving the protagonist's mother, Youta actively aids Koichi in searching for the protagonist and later helps secure medical assistance.

Youta's portrayal includes authentic childhood details, like the realistic way he shuffles backwards out a doorway while putting on his shoes. His name, reversed, forms the Japanese word "taiyō" (sun), creating a thematic contrast with his sister Umi's name meaning "ocean." This naming subtly ties him to the island's natural elements.