Katherine Flower is one of the Flower triplet sisters, alongside Jane and Dominique, originating from the artificial planet Myrus. Falsely accused of crimes by the Pirate Guild to seize control of Myrus, she is imprisoned on the planet Sido.

While captive, Katherine falls under the psychological influence of Crystal Bowie, a high-ranking Pirate Guild member. Manipulated by Bowie, she betrays her sister Jane during Jane's attempt to rescue her, directly causing Jane's death through deception.

After Cobra infiltrates Sido to free Katherine, she remains under Bowie's control. Her actions lead to Cobra's capture and cryogenic freezing. Katherine later states Cobra survived the freezing due to Jane's enduring love, though she remains aligned with Bowie at this time.

Katherine's path shifts following Crystal Bowie's defeat by Cobra. Freed from his control, she assumes her role as the rightful queen of Myrus. In the story's conclusion, she sacrifices herself by diverting Myrus into a nearby sun. This act prevents the Pirate Guild from using the artificial planet as a weapon to destroy the Seventh Galaxy and allows her to reunite symbolically with her deceased sisters Jane and Dominique in death.

Her background establishes the Flower sisters as central figures connected to Myrus, the Pirate Guild's primary objective. Katherine's arc progresses from victim and manipulated pawn to an autonomous actor who chooses self-destruction to protect the galaxy and join her sisters, occurring entirely within the narrative framework of the movie's events.