Shion Hoshino, a student at Keimon West Private High School, pursued aspirations of celebrity fame. Her appearance featured light pink hair typically arranged in a knitted ponytail and dark pink irises. She wore the school uniform—a blue jacket, white blouse, red bowtie, and matching red short suspender skirt—later transitioning to a dark halter, light skirt, and light knee-highs in subsequent appearances. Her recorded measurements were an 85 cm bust (E cup), 58 cm waist, and 80 cm hips.

At Keimon West, Shion observed classmate Shota Doi being bullied by the Takamatsu Group yet refrained from intervening. She and her friends routinely ate lunch on the school roof despite the ongoing harassment, with Shion openly stating she perceived no issue with their conduct. She later described Shota as "pitiful" while watching him in class. Due to work obligations, Shion frequently departed classes early, including one occasion when Shota intended to explain his medical-condition-related absence.

Five years later in 2045, under Karen Kamiya's directive, Shion transferred to Shota's academy to join a repopulation-focused mating program. She voiced frustration that this societal shift obstructed her celebrity ambitions. Karen tested Shion's commitment to Shota by ordering her to traverse the school in only panties, followed by lingerie the next day. During a lingerie-clad class session, Shota kissed Shion and summoned her to the rooftop, where they engaged in sexual activity despite her initial reluctance.

Shion later attempted to flee the academy but was captured by Karen. In their confrontation, Shion challenged Karen's motives for the breeding initiative and inquired about alternative reproduction methods. Karen confirmed alternatives moments before terrorists assaulted the school. Shion's fate post-attack remains unrecorded.

Her given name combines "汐" (shi: eventide, tide, salt water, opportunity) and "音" (on: sound). Her surname "星野" (Hoshino) translates to "star field."