Millie is a character from appearing Pokémon: Advanced Challenge.
Season 7: Advanced Challenge
Millie arrived to the Weather Research Institute to take the job. Ash thought she was Brodie in disguise, who impersonated Millie. However, Brock flirted with her, assuring she was not a man in disguise. Bart was glad and showed her the place she could work, to copy info into the computers. Despite the hard work, Millie took the job.