Three years prior to the main events, the town of Elvania suffered a devastating demon assault. Millie Eunice was rescued by the legendary hero Shion during that attack. Driven by gratitude and the desire to restore her home, she used the reward money she received to train intensively. Her efforts paid off, and she eventually became a hero in her own right, dedicating herself to the reconstruction of Elvania.

Millie is a teenage girl with long blonde hair and blue eyes. She is the younger sister of Yuna Eunice, the childhood friend of protagonist Touka Scott. As a hero, she possesses notable combat abilities, though she joins the story later than the core group. Her motivation is rooted in the trauma of the demon attack and her determination to protect others from suffering a similar fate. She takes quickly to her role as a new hero, embodying the attributes of a dependable younger sister figure within the narrative. Her development revolves around growing into her responsibilities and proving herself as a capable ally in the battle against the demon threat.