Madoka Kei is a character from the anime series Star Driver. She first appears as a transfer student who joins Southern Cross Academy's high school division, first year class two, alongside a fellow transfer student named Kou Atari. She is a member of the Glittering Crux Brigade, where she holds the code name Window Star. Despite her quiet and refined outward appearance, which suggests a polite and reserved young lady, her true personality is markedly different. Fellow Glittering Crux member Kanako Watanabe describes her as really dangerous, indicating that she possesses an extreme and unpredictable nature.

Her background includes a past affiliation with an organization called Adult Bank before she becomes involved with the Glittering Crux. As the story progresses and the Glittering Crux reaches its third phase, Madoka later aligns herself with the group known as Vanishing Age, showing a shift in her allegiances. Her motivations are tied to her role within these organizations, as she participates in the conflict centered around the Cybodies, giant mecha that are summoned in a dimension called Zero Time.

Madoka Kei pilots a Cybody named Hegent. In battle, her Cybody wields a Star Sword called Perle, and she has the ability to create and manifest illusions within Zero Time, a power that makes her a formidable opponent. Her role in the narrative is that of an antagonist, but her trajectory includes a change in faction, reflecting a degree of development as she navigates the larger struggle between the Glittering Crux and the main characters. Her relationships with other characters are defined by her membership in the Glittering Crux and her interactions with fellow members like Kanako Watanabe, as well as her connection to Kou Atari, with whom she transferred. Throughout the series, she demonstrates intelligence and competence in combat, utilizing her Cybody's powers to challenge the protagonists.