Rinna Sawaki arrives as a second-year transfer student at Takamizuka High School, moving directly into Tsugumi Dormitory. She sports short brown hair with spiky bangs, garnet eyes, and a slim 161 cm frame. Her first encounter with the protagonist occurs in a bar while she is intoxicated; after conversing, they depart together. The next morning reveals her in his room wearing only undergarments. When he fails to recall the previous night, she slaps him and escapes through a second-story window.

She maintains an aloof, rude attitude toward classmates, teachers, and dormmates, rejecting a welcome party thrown in her honor. This stems from her refusal to form attachments, anticipating inevitable separation when all dorm residents depart the island within a year due to the local aerospace industry's closure. Her avoidance is directly challenged by others.

Her guarded nature partly originates from the protagonist not recognizing her after their initial meeting. Learning this, she resolves their conflict through a marathon: her victory demands his dorm expulsion, while his win requires her participation in communal activities. Though the protagonist collapses exhausted and takes an illicit shortcut to finish first—resulting in disqualification—she accepts the outcome. This allows her to lower emotional barriers, join dormitory life, and attend a subsequent welcome party.

Athletically inclined, she frequently appears in school uniforms, sports bloomers, or thigh-high stockings. Her tsundere personality blends initial coldness and abrasiveness with deep vulnerability about transient relationships and abandonment fears.