Millie is a narrow gauge engine. She has worked on the Earl of Sodor’s Estate Railway for many years. She is very loyal to the Earl and her relationship is like that of a personal servant. They have fun together and are as close as many friends, but there is also a certain formality to their relationship. Millie is a youthful, fun-loving, friendly engine who is full of spirit, and can be fiery when crossed. But she is also fair-minded and never spiteful. That said, she will take it upon herself to teach an uppity engine a lesson, should she feel the need. She always wants to do the best by the Earl and will defer to his judgement in all things, but when it comes to other engines, she has a mind of her own.
Millie runs the estate railway for the Earl.