Mystradie is a thief who targets the rich and well-to-do as they travel.
She has a run-in with Celica when she notices Rinsha looking at the gem he gave her as collateral for his rooms at the Bison's Hoof. Sensing a profitable mark, she sneaks up to his room to rob him blind, but is disappointed to find that he has nothing else of real value - even his money is no good in Milphe.
She's also a lesbian, and when Suri walks in on her while she's poking around in Celica's room, Mystradie takes the opportunity to have some fun playing with her. This doesn't go over well with Suri, whose bond with Celica alerts him at once in the common room below. He comes up and returns the favor to the would-be thief, while at the same time placing a magical binding on her, making her literally incapable of breaking the law again in the future.
Battle Goddess Memoria
Mystradie does not appear in the Battle Goddess remake, although a scene similar to hers is used to introduce Valefor as a character.