Shirin Reyes is an agent with the Agency.
Exactly which agency is not mentioned. For that matter, exactly what country the Agency for whom she works is not mentioned either. I have listed the nationality of Reyes as being Australian based only on the slimmest of intelligence and could easily be incorrect. One of those sketchy facts is that her boss held the rank of Director of Counter-Intelligence and when having a discussion with a high-ranking government official obviously overseeing his department, he referred to her as Minister. Throw in the fact that the person doing the storytelling (i.e., the author) is Australian. Like I said, sketchy.
From the moment we meet her in the prequel, Reyes, said to be in her late 20's, is a rogue agent. She had more than enough reason. We learn in the first page of that adventure that eighteen months before, her husband Harry, a federal agent, was murdered on the job. According to her, he suffered before his death so if his murder had not been enough to put her in the frame of mind we find her, that would have.
Reyes is slowly but surely working her way up the chain of command, starting with the one who furnished the weaponry used, moving on to who handled the contracting, all to find eventually the top man or woman. Those along the way are going to die but she is giving them, usually, a choice of easy or hard. She does not care which one they choose. The result will be the same.
Prior to her new purpose in life, she had been one of the best agents the Agency had in the field. As one of her immediate superiors remembers of her, "Reyes had been an unparalleled agent when she had worked for him. A pain in the ass, crazy as hell, and the source of many headaches, but she never failed, no matter the cost. In his world, that level of success was all that mattered. Regardless of rules, laws, or intelligence protocols, success warranted certain freedoms. Freedoms that he had readily provided her."
This level of skill is vital to her staying alive for once she starts down the road to learn who had betrayed them, the powers that be against her begin to throw everything and then some at her to put her down permanently. She is not very accommodating to them.