Kirsten Stewart is an agent with the British Secret Service.
Exactly which agency she is employed by is not mentioned; we are given a handful of references solely to her being a part of the "secret service" with no name attached. What we are told is that it operated clandestinely as a matter of course but also that it dealt usually in stopping major criminal activity and not spy-hunting per se. That is not to mean that some of the latter is not given to her but more that her targets were serious law-breakers of a domestic nature.
One interesting description of the organization calls it "a group that investigated the crimes that were beyond what the police had scope to handle, those which required a more direct method of operation. Often, they would work in the dark, taking out threats that if handled in the normal way, would never be neutralized in time before significant events occurred". A case in point is an assassination attempt on the Prime Minister.
She works under the direction of Anna Hunt but she frequently has to deal with a senior officer in the department named Justin Chivers, the man who gave her the codename of Angel, for which she has yet to find forgiveness. Stewart has considerable respect for both, but she finds dealing with Chivers, no matter how smart and efficient and brilliant he might be in his preparation for an assignment, less than thrilling. This includes the way his eyes "always scanned her". Stewart was not worried about his lecherous ways so much as annoyed by them; knowing that he likely would move to a touching stage, she had quickly decided that one he laid a finger on her, "she'd take that finger and bend it back until it snapped".
Prior to coming to work in her current capacity, Stewart has been employed as a police officer, advancing to the position of homicide detective where she had "seen plenty of bodies and a myriad of people with reasons to kill". Nowadays, the things she did were far less out in the open, consisting of "covert surveillance for the main, gathering intelligence, understanding what the situation was, and then moving in for a swift operation".