Mizuki Segawa (瀬川瑞紀, Segawa Mizuki) is a typical schoolgirl who is the little sister of Tetsuro Segawa. She mentions that she is the accountant for the student council. In one of Sho's flashbacks during OoC we learn that they have known each other since childhood.
At the beginning of the story, she has a crush on Agito Makishima, but later begins to appreciate Sho more. She is somewhat short even for a Japanese girl, very slender and wears her hair in a page-boy cut that emphasizes her large eyes.
Initially ignorant of the immense happenings around her, she is shocked at first and downright horrified later to learn of the true nature of the world and humanity. She comes very close to a mental breakdown at the revelation that humans were originally created by aliens as biological weapons. But, just like Sho himself, she overcomes her fears, accepting reality and standing with her friends as well as she can. Her relationship with Sho develops quickly under the extreme circumstances of her life after learning of Chronos and the Guyver; initially, she thinks of him as just her goofy neighbor and childhood friend, but she grows to love him more and more and trust in his power completely. Eventually, their relationship moves into the physical as well, and she and Sho may be considered lovers.
Although Mizuki possesses no 'special' abilities herself, her emotional connection to Sho has compelled him to overcome his perceived weaknesses more than once, eg. when he had suppressed his ability to become the Guyver because of the trauma of his father's death, the sight of Mizuki being tortured by Aptom let him regain his power.
Mizky Segawa is the live action counterpart of Mizuki Segawa in the live action films The Guyver and Guyver 2: Dark Hero live action films. She does have some differences from her Manga, OVA, and Anime versions:
- In the first film, one difference is the spelling of her name. The "U" and the second "I" are removed and in the place of the second "I" is a "Y".
- She has a much more larger damsel in distress role, by being held hostage by the Zoanoids a lot more oftenly.
- Not much of her back story is known, besides the fact that her father, Tetsu Segawa, was killed by Lisker.
- She isn't a high school student but a college student.
In the beginning of the second film she breaks up with Sean Barker because he's killing criminals on the streets as the Guyver and he still thinks Chronos and the Zoanoids still exist and won't forget about them.