Although she dresses in a manner appropriate to her station, she is remarkably ordinary in appearance and manner, and has occasionally been mistaken for a servant of some kind by unobservant strangers.
She is a powerful enchantress in her own right, but is not as often called on to demonstrate her power.
"Millie" is not her real name - or rather, it is her real name, but wasn't until she adopted it at the end of The Lives of Christopher Chant. Who she was before that is a spoiler.
(Note: Some editions of Charmed Life spell her name as "Milly", but "Millie" is officially the correct spelling.)