Mizuki Makabe is a seventeen-year-old idol who debuted as part of the 39 Project. She is a high school student from Kanagawa Prefecture. To those who do not know her well, she appears to be a quiet, calm, and expressionless girl. Her default countenance is a poker face that rarely betrays any emotion, which often leads others to mistakenly assume she is cold or disinterested. However, this is far from the truth. Mizuki possesses a deeply emotional and passionate interior. She simply struggles to display her feelings outwardly and is often unsure how to convey what she is thinking in a way that others can easily read. She is aware of this difficulty and views it as a personal shortcoming. Her desire to overcome her inexpressive nature and become better at sharing her emotions is a significant motivation for her as she pursues her idol career. She worries that her blank expression might cause misunderstandings in serious situations, which sometimes makes her hesitant to speak up. Despite her own concerns, those around her at the theater accept and appreciate her exactly as she is.

Mizuki is a serious, diligent, and well-mannered young woman who takes her responsibilities earnestly. Her earnestness can occasionally lead to unintentionally comedic moments. She has a straightforward and precise way of speaking, which sometimes results in deadpan remarks or awkward attempts at jokes that she herself admits she is not very good at. Beneath her composed surface, she is actually quite earnest and eager. She is a hard worker who puts great effort into her idol training and studies alike. She is academically inclined; she was a member of her school's student council and has a fondness for activities that challenge the mind, such as crossword puzzles. She is also highly dexterous and skilled with her hands. Her hobbies include practicing magic tricks and juggling. She is adept at baton twirling, a skill she cultivated during her middle school years, and learned to play the piano at a young age. Her manual dexterity extends to crafting; she is capable of designing and building elaborate props, such as a Halloween costume piece that she constructed from the design stage onward. She also has a small puppet named Little Mizuki, which she uses in her magic performances.

Mizuki has a nuanced relationship with her own identity. She strongly associates her school uniform with a sense of comfort and stability, and she chooses to wear it even when not required to. She views her idol costumes as a way of presenting a different version of herself. She is also conscious of her slender build, which she has expressed a desire to develop into a more mature figure, sometimes jokingly attributing her hopes for future growth to her mother's physique. This sensitivity about her appearance, alongside her poker face, are two areas she privately wishes to improve.

In terms of her role in the larger group, Mizuki is often a steady and dependable presence. As one of the older members among the newer idols, she naturally steps into a supportive role. She takes genuine pleasure in being relied upon by her younger peers, and she has been shown helping them with their studies and offering guidance when needed. This sense of responsibility is something that visibly motivates her. At the same time, she is also a member of units that highlight different facets of her personality, including the energetic group Otome Storm and the cool-themed unit ES cape. Her reaction to being called part of a cool-type unit was one of barely concealed joy, revealing how much she values being seen in that light.

A defining characteristic of Mizuki is the contrast between her stoic exterior and her inner world. While her face may not show it, she holds great affection for her fellow idols and cares for them deeply. She is often seen performing small, considerate actions for others. Her development across the story is gradual but clear: she becomes more willing to try expressing herself, even if her delivery remains deadpan, and she learns to accept that her colleagues and fans appreciate her for who she is, limitations and all. Her exceptional skill at poker, a point of mild irony given her difficulty with hiding her emotions in daily life, and her proficiency with word games further underscore her sharp, analytical mind. She also has a talent for noticing small details about the people around her. Overall, Mizuki is an idol who is learning to bridge the gap between her quiet disposition and her vibrant inner spirit, channeling her earnest feelings into her performances and her relationships with those at the theater.