Mizuki Segawa is a high school student and the romantic interest of Sho Fukamachi. She is also the sister of Tetsuro Segawa, whose absence in "Guyver: Out of Control" results in Mizuki taking a more central role in Sho's motivations.

Mizuki accompanies Sho when they discover the Guyver unit. Witnessing a Zoanoid attack causes her to faint, leaving her unaware that Sho merges with the unit to become Guyver I. Chronos subsequently targets her, kidnapping and torturing Mizuki to lure Sho. Footage of her abuse is shown to Sho, compelling him to embrace his Guyver abilities for her rescue.

Her captivity forms a significant part of the narrative. Chronos holds her hostage at their underwater base, where Genzo Makishima uses her as leverage against Sho. Guyver III rescues her during the climactic battle, killing Makishima and returning her to Sho. Mizuki awakens in Sho's arms while he is still armored. She pleads for him not to leave, but he departs, resulting in an ambiguous separation. The story concludes with Mizuki back at school, visibly yearning for Sho amid reports dismissing the Chronos base's destruction as an industrial accident.

Compared to other adaptations, Mizuki's role in "Out of Control" amplifies her damsel-in-distress portrayal. Tetsuro's absence shifts her function solely to being Sho's emotional anchor, and her character lacks the broader development seen in manga or TV series, where she gradually reciprocates Sho's feelings and confronts the truth about Cronos. The OVA excludes her initial crush on Agito Makishima and her student council role, focusing instead on her victimization and symbolic importance to Sho's actions.