Mizuki Segawa is the younger sister of Tetsuro Segawa and a childhood friend and neighbor of Sho Fukamachi, with their families sharing a long history. She attends Narisawa High School alongside Sho and Tetsuro, holding the position of student council accountant and later advancing to vice-president. Physically, she is somewhat short and slender for a Japanese girl, sporting a page-boy hairstyle that accentuates her large eyes. Initially harboring a crush on student council president Agito Makishima, Mizuki remains unaware of Sho's romantic feelings for her. She is also kept ignorant of the Cronos organization, Zoanoids, and Guyver units, with Sho and Tetsuro fabricating excuses for their absences to shield her. Her innocence ends abruptly when a Zoanoid kidnaps her, leading to her rescue by both Guyver I (Sho) and Guyver III (Agito). The trauma results in hospitalization, and despite Tetsuro's explanation framing the events as a movie shoot, she remains skeptical and presses him for the truth. Subsequent kidnappings by Cronos, including one alongside Sho's father Fumio Fukamachi to the Relic's Point facility, force Mizuki to confront the horrifying reality of Cronos's experiments and Sho's identity as Guyver I. Witnessing Zoanoid creation and learning Sho has been protecting her causes deep distress. Her despair intensifies upon learning Sho was compelled to kill his own father after Cronos transformed Fumio into the Zoanoid Enzyme II, and worsens when journalist Masaki Murakami reveals humanity's engineered origins as biological weapons for an alien race. This revelation nearly shatters her mentally, causing her to flee into a forest. Sho follows during her escape but cannot transform due to trauma, leaving them vulnerable. Discovered by Zoanoids, they are saved by the renegade Lost Number Aptom, who reveals himself as Guyver III. Aptom subjects Mizuki to psychological and physical torture, slashing her clothes and threatening her life to force Sho to overcome his mental block and transform. Witnessing Sho's anguish and determination to save her enables him to regain his Guyver abilities. This pivotal experience shifts her perspective, making her realize Sho's profound feelings and leading her to reciprocate his romantic affection. Her perception of Agito alters drastically after observing his ruthless slaughter of enthralled Zoanoid villagers in Takeshiro, whom he deemed beyond salvation. This eradicates her previous romantic interest in him. Following Relic's Point's destruction and Cronos's global takeover (X-Day), Mizuki, Tetsuro, and their allies enter hiding. Believing Sho survived the bio-ship's destruction, she waits a year for his return, though her hope strains under the pressure. In desperation, she visits her old home and discovers teru-teru bozu dolls resembling her and Tetsuro hanging by the door, interpreting them as her parents' hope for their safety. Inspired, she crafts a similar doll for Sho, refusing to abandon hope. Her faith is rewarded when Sho reappears during a Tokyo battle, emerging in the powerful Guyver Gigantic armor. In the live-action film adaptations, the character "Mizky Segawa" differs significantly. She is portrayed as a college student rather than a high school student. Her role emphasizes the damsel-in-distress trope, frequently held hostage by Zoanoids. Her backstory is simplified, primarily noting her father Tetsu Segawa was killed by Lisker. In the second film, she breaks up with Sean Barker (the Sho Fukamachi counterpart) due to his vigilantism as the Guyver and obsession with Cronos.