Sunny, an orphan from Peru, lost both parents before photographer Toshihiko Hazuki adopted him during his work there. He became Toshihiko's son and You Hazuki's adoptive younger brother, moving with the family to Tokyo. Sunny developed a deep attachment to You, persistently following her everywhere, including school and her volleyball practices and matches. This behavior sometimes caused disruptions, notably an early incident where he interrupted a crucial match by wetting his pants on the court, creating friction between You and team captain Nami Hayase.

His arrival coincided with Toshihiko's opposition to You's volleyball ambitions, partly linked to unresolved family history involving You's mother. Their early interactions were significantly conflictual; angered by teasing about Sunny's fear of swimming, You once hung him from a high window and threatened to drop him unless he agreed to learn. The intensity of this scene led to censorship in some European adaptations.

Despite this harsh beginning, their relationship evolved. Sunny consistently viewed You as his "Cool Big Sis," maintaining admiration and loyalty through their difficulties. Primarily, his character influenced You's personal growth and family dynamics, often serving as comic relief or a catalyst for interpersonal conflict. He occasionally demonstrated some ability in English within the series.

Sunny does not appear in the sequel series "New Attacker You!," which shifts focus to China years later, featuring older versions of You, Nami, and others. His story remains entirely within the original 1984-1985 anime narrative arc.