She is the six-year old great, great, great, great, great granddaughter of the 13th President of the United States, Millard Fillmore. She only briefly appears in the episode "The Rotwood Files" (Season 2) and never makes another appearance in the series.
Millie is a short little girl that reaches no taller than an average teenager's waist. She has short, curly blonde hair and braces on her teeth. Her usual attire mainly composes of a red shirt and a white bow on top of her head.
When performing for the Fillmore Fest, Millie wears an outfit inspired by the colors of the United States. It is composed of a blue sleeveless dress sprinkled with white stars worn over a red-sleeved shirt. A white cowboy hat, a tie, gloves, and a pair of boots (all pure white) serve as her accessories. The American flag is also tied around her neck. While onstage, Millie drags around a white stick horse with a red mane.
Millie is a cute little girl that naturally acts in an adorable way. She constantly smiles sweetly, but will easily lose her positive attitude when upset. She can also act like a brat, as shown when she cries loudly and kicks Professor Rotwood in the shin when he snapped at her for coming on stage too early (although this was his fault to begin with).
Because of her affiliation with Millard Fillmore, Millie was invited to the Fillmore Fest during "The Rotwood Files", a celebration usually held every Fall at Millard Fillmore Middle School in honor of its founder. She was supposed to sing to the crowd on Professor Rotwood's cue. However, after being pranked and to that effect causing mass confusion, Rotwood accidently cues Millie too soon, thus ruining the celebration.
- (singing) "Yankee Doodle went to town riding on a pony..."
- According to Rotwood, Millie has a very melodious voice.
- The only sentence Millie says in the entire series is a verse of the United States' Yankee Doodle song.