Grace Flint is a policewoman, not a spy.
At least, she is not a spy initially. As the series begins, her specialty, and what made her so successful in the department handling Major Crimes, was her ability to go undercover and truly become whatever her role was. She could blend in with the best, pushing her true feelings so deep that even she had a hard time finding the truth.
Then a case went horribly bad through no fault of hers and her supervising partner was killed and she felt her face be pummeled relentlessly, leaving her horribly disfigured. Months of painful reconstructive surgery were necessary to restore her to her original beauty and the skillful surgeons did just that. But restoring her confidence would take considerably longer.
To help her return to active duty, she was transferred to a liaison position in America working with many of their various agencies: FBI, ATF, IRS, Secret Service, and some with other initials. In this capacity, she found her work had changed from being a policeman to being more of a secret agent as the cases took on an international aspect.
Flint is a hard woman who doesn't really want to be. Life has not been kind to her, though, and she has to fight to survive. Luckily, she is good at fighting. But when the one you are fighting against turns out to be the one you loved and thought loved you, it's hard to pick yourself up and go on. Flint is able to do so, reluctantly, perhaps, but still determined. Flint is among many other admirable traits one heck of a survivor.
But just as it seems that maybe the Fates are taking a bit of pity on her and letting up some, her career becomes even more demanding.