Millie was born to a businessman father and a native American mother. From an early age she became friends with Lilly Baxter, Melanie Ravenswood, and other children from the upper classes. At age 8, she and her entire family mysteriously disappeared from Thunder Mesa, without any explanation as to where she went.
11 years later, she returned to the town alone, and began to work her way back up the social ladder, and attempting to reconcile lost friendships, while learning more about the local native American tribe, due to a fascination with its deity, "The Thunderbird". She also seems to have a superiority complex, asking everyone to address her as "Lady Ravenscroft".
Millie can be stubborn and docile mostly, but can have a fiery temper if agitated or annoyed. She can also be extremely obsessive regarding items and people, which could point to her possibly having Obsessive–compulsive disorder.