Yuna Kamihara is a Japanese elementary school student who begins the series at age 10 in fourth grade and turns 11 during fifth grade. She lives with her grandmother Obaa on the fictional Izayoi Island in the Ryukyu Islands near Okinawa. Her father works remotely as a marine biologist around Okinawa, while her mother perished in a typhoon during Yuna's infancy. Trained in karate by her late grandfather, she carries special moonsand he gifted her as a good-luck charm. Yuna's personality blends sweetness and outgoing charm with tomboyish toughness, feistiness, and an overbearing sense of justice that drives her to confront wrongdoing. She consistently earns low mathematics scores but shows deep affection for porpoises or dolphins. Her name derives from the Okinawan sea hibiscus flower.

Yuna's life transforms when she bonds with Stitch, an alien experiment crash-landed on Izayoi Island. They become inseparable best friends, exploring Chitama Forest—home to magical yōkai—and discovering the Chitama Spiritual Stone, a wish-granting artifact requiring 43 good deeds. Yuna aids Stitch in collecting deeds to become the universe's strongest being, though he later abandons this goal to stay with her.

Midway through the second season, Yuna's cousin Tigerlily Sakai arrives, treating others kindly but subjecting Yuna to harshness and blackmail over chores, claiming to model responsibility. After Izayoi Elementary closes in the third season, Yuna and Stitch relocate to Okinawa New Town with Tigerlily, whose demeanor softens. Yuna navigates rivalries with Penny, a wealthy girl whose family owns pineapple plantations and resorts; Penny frequently mocks her, leads a rival karate team, and clashes with Yuna despite rare cooperation. Yuna also befriends Kijimunaa, a timid yōkai, by helping reclaim his tree from a bully.

Yuna shares her grandmother's belief in yōkai, a foundation for encountering supernatural challenges tied to the island. In Okinawa New Town, she attends Kaiei Elementary while Tigerlily transfers to a nearby school. Tigerlily's junior-high connection to twins Mitsuki and Hazuki integrates Yuna into new social circles. Both cousins retain an aversion to thunder.