Princess Mill Varna hails from Bartonia, a fantasy kingdom where her ruling family was slain during the Jaina holy group's violent takeover. Forced into fugitive status, she evades relentless pursuit by Jaina soldiers intent on her elimination. Mill exhibits an outgoing, childlike personality, frequently punctuating speech with "ukyuu." She harbors profound abandonment issues, particularly concerning her protector Maze. Inheriting a royal ability crucial to her survival, Mill summons the powerful Demi-Armor mecha Dulger—though she cannot pilot it herself.

After Maze's falling house accidentally crushes her pursuers, Mill forms an intense attachment. She addresses Maze as "Onenii-sama" (Big Sister-Brother), acknowledging Maze's dual-gender existence. Mill openly expresses romantic and sexual interest in both Maze's forms: discussing marriage with the female version and procreation with the male. Her fixation includes physical possessiveness and detailed scenarios ensuring Maze remains permanently with her. Mill's objective shifts from survival to reclaiming her throne. She becomes central to the war against Jaina, leveraging Dulger's power and Maze's protection to restore Bartonia's sovereignty.

Supplementary materials—light novels, manga, and an unaired episode (26)—establish Mill as intersex (hermaphrodite), adding complexity to her character. Evolving from a vulnerable princess into a determined leader, Mill channels her initial obsessions into focused command. Her bond with Maze remains foundational, driving her actions to defeat Jaina and regain Bartonia's independence.