As a young Watchog, Millie arrived at Unova's Nature Preserve during its time of need. A giant gray Haxorus had been terrorizing the residents of the Preserve, and all of the rangers' efforts to dispel its presence had failed. Millie was enthralled by the grace and devotion of the rangers, and begged to be part of their forces. Though most of the Pokemon were reluctant to allow her to enter, an older Zebstrika who was widely respected by the community agreed to take her under his wing and train her. 

In a moment of utmost importance, Millie was able to prove herself as the competent ranger that the Zebstrika had trained her to be-- while out training alone, she was cornered by the Haxorus, but she managed to lasso its head and snap off one of its blades, leaving it to flee in shame and allow the Pokemon of the nature preserve to live in peace once more. The incredible feat led to her rising rapidly through the ranks of the rangers, and in just a few years she was instated as the new Head Ranger and overseer of the park. She's incredibly passionate about her work; as human development takes over more and more of Unova, she tirelessly educates the masses on why the preservation of beautiful natural scenery is so important.