Mizuki Segawa is a seventeen-year-old high school student who serves as a major supporting character in the anime The Guyver 2. She is the younger sister of Tetsuro Segawa and a childhood friend of the protagonist, Sho Fukamachi. Initially living a normal, sheltered life, Mizuki is unaware of the secret conflict involving the Chronos Corporation and the alien-derived Guyver units. When she is drawn into this violent world, her initial reactions are shock, fear, and horror; she comes close to a mental breakdown upon learning that humanity was originally created by aliens as biological weapons. Over time, however, she develops greater resilience, though she never becomes aggressive or physically combative.

Mizuki is portrayed as demure, quiet, and naturally domestic, with a gentle and caring disposition. Her primary motivation is the safety of those she loves, especially her brother Tetsuro and Sho. She begins the story nursing a crush on the aloof and manipulative upperclassman Agito Makishima, who only sees her as a tool to control Sho and Tetsuro. As events unfold, Mizuki gradually realizes that her true feelings are for Sho, and their relationship deepens into a sincere romantic bond built on trust and mutual protection. She becomes a vital emotional anchor for Sho, giving him the strength to continue fighting despite his own doubts.

In the story, Mizuki often occupies the role of a vulnerable civilian who must be rescued, placing her in direct danger from Zoanoid attacks. On one occasion she is captured and threatened by the Zoanoid Aptom, an ordeal that leaves her traumatized but also reinforces her resolve. Her relationship with Sho develops quickly under these extreme pressures, transforming from a simple childhood friendship into a profound love. She also maintains a close, supportive bond with her brother Tetsuro, who alongside Sho works to protect her. While Mizuki possesses no superhuman abilities, her notable qualities include her emotional resilience, her capacity for kindness, and her ability to inspire hope in others through her steadfast presence. Her growth throughout the series is marked by a transition from a frightened, passive observer to a more self-possessed young woman who confronts the terrifying reality around her without losing her essential warmth.