Thirteen-year-old Mitsuki Hoshikawa begins her first year at Paradise Private Academy as a central figure. Sharing a dorm room with her childhood friend, Himari Aozora, Mitsuki presents a reserved and serious demeanor. Her physical traits include curved blue eyes flecked with pale yellow and blonde hair styled with bangs predominantly swept right, featuring a distinctive longer lock on the left. Performance triggers a dramatic transformation: her hair extends to thigh-length, accented by light blue and purple highlights, while small curled locks appear near each ear; her eyes brighten, complemented by light makeup.

A blend of caution and responsibility defines her personality, leading her to secretly enter the virtual Aipri Verse alone to gather knowledge before Himari could join. Despite her usual composure, she harbors an intense fear of ghosts, causing immediate fainting spells upon encountering anything supernatural. Talented in piano and baking sweets, her anger manifests as frightening intensity concealed behind a superficial smile.

Relationships are central to her journey. With Himari, she shares a profound bond, offering emotional support through a "good luck charm" ritual involving a forehead tap. Their friendship strains when both violate a childhood pact by pursuing parallel careers as Aipri idols in secret. Mitsuki had debuted earlier under the stage name "Mi-chan" to pave the way for Himari; the eventual revelation of this secret fosters mutual understanding and reconciliation. They later collaborate in the groups Secret Friends∞ and Poppin' Dreamin'. Her dynamic with Tsumugi Suzukaze starts conflictually as rivals for Himari's attention but matures into mutual respect and friendship. She admires Sakura Ichijoji's dedication as student council president and fellow idol, though Sakura's fervent admiration occasionally embarrasses her.

Mitsuki's family includes her older brother, Yakumo Hoshikawa, grandparents Seiun and Kei Hoshikawa, and a pet cat named Comet. Her surname, "Hoshikawa," translates to "star river" ("hoshi" for star, "kawa" for river), while "Mitsuki" means "beautiful moon" ("mi" for beautiful, "tsuki" for moon).

Progressing into her second year at Paradise Private Academy for the series' second season, Mitsuki faces new challenges. The announcement of an "AiPri Grand Prix" by Princess Ring Clover, promising the victor the title of "Eternal AiPri," sparks heightened enthusiasm. She navigates this alongside Himari and expanding friendships, uncovering new mysteries tied to Princess Ring's secrets.

Her trivia includes a preference for curry rice as her favorite food and milky blue as her theme color. Her performance repertoire features solo, duo, and group songs like "Secret Dream," "UNBEATABLE DUO ENERGY," and "Shiny My Story."