Myoudouin Itsuki serves as student council president at Myoudouin Academy Junior High School. She is the granddaughter of the academy's principal, Myoudouin Gentarou. Because her brother Satsuki's chronic illness prevents him from fulfilling heir duties to the family dojo, Itsuki cross-dresses and adopts masculine mannerisms. She wears the boys' uniform and uses the pronoun "boku," leading to frequent misgendering, even by those aware of her situation. Her Heart Flower is the peony, symbolizing rebirth and nobility.

Initially, Itsuki suppresses her fondness for cute, girly items like toys and fashion to maintain her princely image, hiding a plush bunny received from an admirer. When Desert Apostle Cobraja damages this doll and steals her wilting Heart Flower, her internal conflict manifests as a Desertrian born from a bronze statue. After Cure Blossom purifies her Heart Flower, Tsubomi invites Itsuki to join the Fashion Club as a path toward self-acceptance, though she hesitates.

Itsuki gradually embraces her identity through designing clothing with Tsubomi and Erika. She creates a gold tunic with orange accents, denim leggings, and white sneakers—a gift from her friends symbolizing her integration of masculinity and femininity. Her brother Satsuki affirms her choice, calling the outfit cute. This acceptance allows her to later switch to the girls' school uniform and grow out her hair.

Her protective love for Satsuki awakens her transformation into Cure Sunshine when his Heart Flower is targeted. She chooses this name after seeing the sun, inspired by Satsuki's past encouragement. Her transformation phrase is "Pretty Cure! Open My Heart!" As Cure Sunshine, her short brown hair becomes long golden twin-tails, her eyes turn gold, and she wears a cream and gold two-piece outfit with orange ribbons, a cyan heart brooch, and flower motifs. She introduces herself with "The flower bathing in the sunlight, Cure Sunshine!" Her battle catchphrase is "I will heal the darkness in your heart with my light!"

Cure Sunshine utilizes the Shiny Perfume instead of Flower Tacts and the Shiny Tambourine for attacks. Her primary technique is Gold Forte Burst. She participates in combination attacks like Shining Fortissimo with Cure Blossom and Cure Marine, and Floral Power Fortissimo with Cure Moonlight. Later, she achieves Super Cure Sunshine form, gaining longer pigtails, a heart-shaped tiara, rainbow light wings shaped like hearts, and enhanced abilities.

Itsuki maintains key relationships: Satsuki remains her inspiration; her fairy partner Potpourri bonds over shared protective instincts; Tsubomi transitions from a brief romantic misunderstanding to close friendship and fashion collaboration; Erika evolves from club dissolution tensions to supportive camaraderie. She also develops mutual respect with the initially distant Cure Moonlight, Yuri.