Mitsuki Hoshikawa is one of the two main protagonists of Himitsu no AiPri: Ring-hen, the second season of the series. She begins the season as a second-year junior high school student at Paradise Private Academy, having advanced from her first year alongside her best friend and dorm mate, Himari Aozora. She is calm, composed, and serious by nature, often acting as the steady and protective presence that balances Himari's more anxious and energetic personality. Mitsuki is a high-achieving student who completes her assignments quickly and reliably, and she has a methodical approach to her goals, keeping a daily journal to track her progress. Her practical and collected demeanor makes her a natural source of support for those around her, particularly Himari, whom she often calms by pressing their foreheads together when the latter becomes flustered.

Beneath her cool exterior, Mitsuki harbors a strong sense of responsibility and a deep loyalty to her friends. She is skilled at playing the piano and enjoys baking sweets, hobbies that reveal a softer, more domestic side that contrasts with her usually serious image. She is notably terrified of ghosts, a comedic trait that highlights her more vulnerable moments. Her athletic abilities are limited, standing in clear contrast to her academic strengths: she struggles with sports such as soccer and vaulting, and she requires extra practice to keep up in physical activities. This distinction mirrors the complementary dynamic she shares with Himari, who excels in areas where Mitsuki is less confident.

Mitsuki's motivations are deeply rooted in her childhood. She and Himari made a promise to debut together as AiPri idols, a secret dream they both held close. In the first season, it was revealed that Mitsuki had already been active as the popular and widely admired AiPri idol Mi-chan, keeping her identity hidden from everyone, including Himari. This secret initially created tension between them, but once they confessed the truth to each other, their bond grew stronger, and they began performing together as the duo Poppin' Dreamin. In Ring-hen, their shared ambition evolves into a pursuit of the Eternal AiPri title, a goal set forth by the mysterious Princess Ring Clover, who announces the AiPri Contest. Mitsuki is excited and highly motivated to compete, viewing it as the next step in her journey alongside Himari.

Her role in the story is that of a dedicated partner and a grounded counterpart to Himari. Within the AiPri Verse, she performs as part of Poppin' Dreamin, and later as a member of the trio Secret Friends Infinity alongside Himari and Tsumugi Suzukaze. In Ring-hen, she also becomes entangled in the deeper mysteries surrounding Princess Ring Clover and the library's enigmatic librarian, Wako-sensei. Mitsuki receives a mysterious ring from a woman claiming to be Ring Clover during her childhood, a piece of jewelry that later proves to be half of a pair that fits perfectly with a ring owned by Wako-sensei. This revelation points to Wako-sensei being the true identity of the legendary idol who mastered all five types of Buzzlume Change. Mitsuki's connection to the ring and her interactions with Wako-sensei place her at the heart of the season's central mystery, giving her a pivotal role in uncovering the truth behind the AiPri Contest and the fate of Princess Ring Clover.

Mitsuki's key relationships are defined by her deep and enduring friendship with Himari, which serves as the emotional foundation of the series. The two are inseparable, and their mutual trust and affection drive much of the narrative. She also shares a strong bond with Tsumugi Suzukaze, their fellow team member, whose mysterious origins and evolving friendship become important to Mitsuki's own growth. Her relationship with Wako-sensei is significant in Ring-hen, as it connects her personal history to the larger lore of the AiPri world. Additionally, Mitsuki maintains a respectful but competitive dynamic with other idols who participate in the AiPri Contest, such as the members of Quartet Star and the newly introduced underclassmen.

Throughout the series, Mitsuki undergoes meaningful development. In the first season, she learned to be more open about her feelings and her secret identity, moving from a guarded perfectionist to someone who could share her true self with her closest friend. In Ring-hen, her growth continues as she faces new challenges, including the pressure of the AiPri Contest, the responsibility of bearing the mysterious ring, and the impending reality of studying abroad, a future plan that adds emotional weight to her present actions. Her journey involves learning to balance her desire to protect those she cares about with the need to let them grow alongside her. She ultimately embraces her role as a partner and a leader, proving that her strength lies not only in her composure but also in her capacity to trust and rely on others.

As an AiPri idol, Mitsuki's preferred brand is Miracle Moon. She is capable of performing Buzzlume Changes, the special transformations that allow idols to enhance their performances with unique costumes and effects. Over the course of Ring-hen, she and Himari unlock new forms, including the Princess Buzzlume Change, a legendary transformation that represents the culmination of their bond and their growth as idols. Her abilities as a performer reflect her disciplined personality, but as the story progresses, she learns to infuse her performances with genuine emotion, allowing her to connect more deeply with her audience and her teammates.