Little is known about Jackie Meyer's history, mainly because she is a private person and hasn't revealed any of those details to any POV characters. I plan on elaborating on more of her past in the final four books of the series, so you will learn much more about her.
What is known is that she is a leader in the Department of Magic. While at first she appears to be one of Josiah's closest supporters, she has been secretly working on a resistance against him. She is instrumental in helping Cadan and the other Paladins escape from the Department.
Jackie is quite possibly one of my most favorite characters in the entire series. I think one of the most intriguing characteristics is that she has that "anything to get the job done" mentality. She wants to stop Josiah, and she doesn't mind killing an enemy to make that happen. She blurs the gray, ambiguous lines of morality. Definitely not your typical "good guy".
Her sole motivation is to stop Josiah. I haven't yet revealed what fuels that motivation though. Stay tuned.
She has stated before in her own POV that there were two men she has loved before. There are clues in the story that reveal both of their identities, but I haven't come right out and stated yet. Never fear, for I will definitely be getting to that in more detail in Book 10.
A common question I get about Jackie is why Cadan is so sure that he can trust her in the beginning. Well, that is actually a fairly big secret that I will be revealing about her. So far I've shown that only Cadan and Jackie are in on this secret, but there are actually several more characters who are aware of this secret. Again, I have put clues in the books to help the reader figure it out, and I hope to write the big reveal in Book 10 or 11 (probably 11).