Miyuki Hoshizora, a cheerful and optimistic middle school student, frequently exclaims her catchphrase "Ultra Happy!" Her energetic nature contrasts with recurring clumsiness, causing trips or collisions during excited moments. Fairy tales became her childhood refuge during shyness and isolation. A pivotal encounter with a mysterious girl and her grandmother's guidance—culminating in a mirror necklace to practice smiling—shaped her outgoing personality.

As Cure Happy, she transforms via the Smile Pact to lead the Smile Pretty Cure team. Her light-manipulation abilities include the primary attack Happy Shower, projecting heart-shaped energy bursts, alongside enhanced physical prowess like high jumps. Her Cure form displays bright pink pigtails, a winged-tiara, and a light pink dress with a magenta bow. Advanced transformations encompass Tiara Mode (golden crown), Princess Happy (gown with upgraded attacks like Cinderella Happy Shower), and movie-exclusive Ultra Cure Happy (featuring wings, elongated hair, and amplified combat strength against adversaries like the Demon King).

Her arc spans multiple narratives: In *Smile Pretty Cure!*, she evolves from uncertainty to a resolute leader prioritizing teamwork. A light novel sequel depicts her young adulthood in a bookstore, mentoring a shy girl, Yoshimi, echoing her past. Crossover films like *Pretty Cure All Stars New Stage: Friends of the Future* feature collaborations with Cures such as Hibiki Hojo (Cure Melody) and Mana Aida (Cure Heart), reinforcing her unifying role.

During *Eiga Precure All Stars: Haru no Carnival*, Miyuki participates in Harmonia Kingdom's spring carnival with other Pretty Cure teams. Villains Odoren and Utaen steal fairies and transformation devices, provoking chaos and enraging the guardian dragon. She aids in retrieving the fairies and confronting the antagonists. After Princess Pretty Cure temporarily calms the dragon using Rainbow Tornado, Cure Happy joins all Cures in a celebratory performance restoring the kingdom's harmony.