Katherine Denise "K. D." Thorne is an agent with the NDA.
That is the National Defense Agency and if you haven't heard of it, do not feel alone as apparently there is a lot of that going around. When Thorne is offered a chance for a job with the American governmental organization with a headquarters in Suitland, Maryland just outside of D.C., she has to ask, "So this really is a government agency?". She learns that it is and that they have use for someone with her impressive talents and without much hesitation on either side, she was officially with the group and out on her first assignment.
According to the NDA's Director, Clara Garcia, the role of the NDA is not intelligence gathering nor is it law enforcement. She puts its mission as "strictly national security investigations" which is a tad nebulous but does open the door for a variety of missions for Thorne and her newly assigned partner, a black man name Blunt she served together with in Iraq.
Thorne might or might not still technically be in the U.S. Army, holding the rank of Captain with the prestigious Rangers. She is at loose ends and able to accept the NDA job offer because of some issues several years back. Issue #1 was refusing to leave behind a man while on a mission; disobeying orders, she found him and carried him home - a noble act but still disobedience. Since that put a stopper on advancement, she laterally slid over to liaison duty with NASA where her doctorate (unknown subject) was appreciated and respected. But then Issue #2 came when she chose to have a brief affair with one of its key personnel and that fellow's wife found out and made a stink and now she is able to seek employment elsewhere.
Thorne is likely in her late-30s at best, having been previously married for ten years to a man she still loves but who she no longer trusts. They are talking again but her defenses are raised high.
She is not likely to be much older, though, because she is still in prime fighting condition as more than a few fellows have found out the hard way.