Rin Natsuki balances her responsibilities as a second-year student at L'École des Cinq Lumières with helping at her family's Natsuki Florist shop and caring for her younger siblings, Ai and Yu, who sometimes frustrate her but are deeply cherished. Her personality merges tomboyish athleticism with feminine interests; she excels in sports like futsal and tennis, attracting attention from school teams, while also pursuing accessory design, with her creations sold at Natts House year-round. Rin displays a grounded, straightforward nature, often acting as a protective older sister figure to her optimistic childhood friend, Nozomi Yumehara, providing pragmatic counterpoints to Nozomi's cheerfulness. Despite her generally level-headed demeanor, she possesses a passionate and occasionally hot-tempered side, leading to early clashes with more reserved individuals like Karen Minazuki. Rin harbors deep fears of losing loved ones, especially Nozomi, and encountering unnatural phenomena like ghosts, a phobia rooted in childhood experiences that required extended recovery periods after frights.

Initially rejecting Nozomi's invitation to become a Pretty Cure due to her busy schedule, Rin's resolve to protect her friend solidified after witnessing Nozomi, as Cure Dream, endangered during a confrontation with Girinma. This emotional catalyst summoned a red butterfly granting her the Pinky Catch, enabling her transformation into Cure Rouge, the Pretty Cure of Passion. As Cure Rouge, her hair turns bright red, styled like flames and secured with a magenta swallowtail butterfly hair clip, while her eyes adopt a matching red hue. Her attire features a cream dress with red accents, butterfly motifs, orange boots, and dark pink shorts. Her fire-based attacks begin with "Rouge Fire," evolving to "Rouge Burning" after obtaining the Rouge Tact weapon; she also contributes to team attacks like "Five Explosion."

Character development sees Rin and Karen overcoming their initial friction; arguments, such as over Natts House decorations, gradually give way to mutual respect after recognizing their shared dedication to protecting others. Rin navigates a brief romantic subplot involving a crush on Momoi Kyosuke, later revealed as the disguised Nightmare member Mucardia exploiting her trust. In "Yes! Pretty Cure 5 GoGo!", her powers and appearance upgrade: her hairstyle lengthens slightly, incorporating a violet rose accessory, and her outfit shifts to a pale pink ruffled dress under a red-accented vest. New abilities include "Fire Strike," where she generates and kicks a fireball, and participation in combined attacks like "Rainbow Rose Explosion." Movie appearances grant temporary power boosts: "Great Miraculous Adventure in the Mirror Kingdom!" features a Super Cure Rouge form with enhanced outfit details and large red butterfly wings, while "All Stars DX 2" showcases a Rainbow Cure Rouge form with a feather-like pale pink and scarlet design, golden wings, and a light pink rose emblem.

Her name carries thematic weight: "Natsuki" (夏木) combines "summer" (夏), referencing her fire powers and the season's heat, and "tree" (木). "Rin" (りん) is written in hiragana, lacking inherent meaning, though it could imply "dignified" (凜) if rendered in kanji. "Cure Rouge" derives from French for "red," aligning with her color motif and the series' linguistic elements. As an adult depicted in later media, Rin's hair reaches her shoulders with a slightly unkempt style. Her attire matures into sophisticated combinations like ruffled beige blouses with ankle-length skirts or layered vests and jackets, often accessorized with gold jewelry and red earrings.