Princess Mill Varna hails from the royal line of either Bestool or Bartonian. Her parents and family perished in the violent Jaina holy group takeover of their territory, leaving her orphaned and displaced, an event fueling abandonment issues. She formed an intense attachment to the protagonist Maze after their first meeting. Mill exhibits an outgoing yet childish personality, frequently using the verbal tic "ukyuu." She refers to Maze as "Oneenii-sama" (オネニーサマ), a hybrid term reflecting her perception of Maze's dual-gender nature. Her obsession manifests in frequent declarations of intent to marry the female Maze while planning children with the male Maze.

Biologically, Mill possesses intersex characteristics, specifically identified as a hermaphrodite in the light novels, manga, and anime episode 26, consistent across official media. Functionally, she serves as the sole summoner of Dulger (or Dulgar), one of the world's most powerful Demi-Armors, though only Maze can pilot it. This ability grants her strategic importance against Jaina.

Throughout all narrative seasons, movies, and OVAs, Mill journeys with Maze alongside companions like Demi-Armor hunters Aster and Solude, the fairy Randy, and knight Rapier. Her primary story arc focuses on reclaiming her throne in Bartonia from Jaina, driving her involvement in the war. While she interacts with the party, her attachment to Maze remains central to her motivations and emotional development.

Her background as displaced royalty shapes her actions and objectives, positioning her restoration as a narrative focal point. Her character design features consistent auburn hair and brown eyes across visual adaptations. Her tragic history, biological uniqueness, and pivotal role summoning the Dulger frame her functional and emotional significance within the series' events.