Mizuno You is a student at Southern Cross High School on Southern Cross Isle. She is one of the four maidens known as the West Maiden, a role that ties her to the island's ancient secrets and the giant mecha called Cybodies. Her own Cybody is named Memna.

Her personality is bright, cheery, and gentle, yet she also comes across as naive, childish, and somewhat eccentric. She often speaks to birds and teaches her classmates what she calls magical spells, adding to her mysterious aura. Despite her innocent demeanor, she occasionally shows a flirtatious side, particularly toward the protagonist Takuto Tsunashi, whom she develops a quick infatuation for. She frequently follows him around, which creates a mild rivalry with Wako Agemaki, another maiden who shares a bond with Takuto.

Mizuno lives with her sister, Marino You. It is later revealed that Marino is actually an alter ego or clone created by Mizuno's own powers, a secret known only to a few. Her mother, who was also a maiden, left the island at some point in the past.

In the story, Mizuno's role is that of a supporting character and a key figure in the larger conflict over the maiden seals. The Glittering Crux, an organization seeking to break the maidens' seals and unleash their Cybodies, targets her. She is kidnapped and forcibly placed in her Cybody Memna, which is then destroyed, breaking her seal. After this event, she decides to leave the island, following the path her mother took.

Notable abilities include her power to communicate with birds and her knowledge of folk magic, though these are presented as quirks rather than combat skills. As a Star Driver, she can pilot her Cybody Memna inside the dimension called Zero Time. After her seal is broken, she loses her connection to the Cybody, but her sister Marino continues to exist independently. Mizuno also demonstrates an unusual amount of luck, such as leaping from a window and landing safely on a moving bus without injury.