Mizuki Tachibana is a sixteen-year-old student at Maihama Minami High School. She is the close friend of Ryoko Kaminagi, with whom she shares many school activities. Mizuki is a member of the theatre club, the tennis club, and the volleyball team. She possesses a straightforward personality, which is evident in her direct approach to her personal feelings. She is also deeply passionate about acting and enjoys participating in the theatre club's productions. Her motivations are largely personal and grounded in her everyday life as a high school student. Mizuki harbors romantic feelings for a classmate named Toya Hayase. Although she is aware that Hayase is in a relationship with Sachiko Tsumura, Mizuki chooses to confess her feelings to him during a summer festival. The confession is rejected, but she does this in order to find closure and move on. Following this episode, she begins to happily date another student named Ushio, and the two share a meaningful relationship until the server reset occurs. In the story, Mizuki functions as a supporting character who highlights the everyday high school lives of the main cast before the larger conflicts over reality unfold. Her key relationship is with her best friend Ryoko, and through her connections to Hayase, Ushio, and the broader circle of friends, she provides a contrast to the characters who directly pilot the Zegapain mecha. Mizuki is one of the individuals whose existence is part of the simulated Maihama server. Her development is most visible in her romantic arc, as she navigates her feelings for Hayase, experiences the pain of rejection, and then finds happiness with Ushio. This progression shows her emotional maturity and willingness to embrace new beginnings even within a seemingly repeating world. Her notable abilities are not related to combat or piloting; instead, she is distinguished by her active social life, her commitment to her clubs, and her direct emotional honesty. Her favorite film is Last Year at Marienbad.