Ethan Warner and Nicola Lopez are agents with the D.I.A.
Sometimes they are. Sometimes they go off on their own together. Sometimes they go off on their own alone. But then they get back together and do stuff for the D.I.A. so ...
When we first meet the pair, Warner is trying to stay away from, well, everything by going off on his own in the West. A former Marine officer in their Expeditionary Forces, he had seen a great deal of very unpleasant action. When his service hitch came up he elected to switch to something else that interested him - journalism. His extensive knowledge of military actions and procedures made him a perfect candidate to be embedded with pretty much the same sort of troops he once led. Then, though, after a few years of this, something terrible happened on one assignment and Warner in disgust shucked it all. He will be pulled back by a former Marine commander now working for the D.I.A.
Lopez is a young but impressive detective with D.C.'s Metropolitan Police, described as petite but no one going against her in a fight, or watching someone get a whuppin' from her would ever think petite meant safe. As she tells one perp she apprehends after a tussle, "You have the right to remain silent, else I kick your sorry ass further." She would, too. She will find reason to leave her career as a police officer and join Warner in contract work with the D.I.A. as well as go into the bail bonding business, especially good at retrieving runners.
The relationship between these two equals will ebb and flow but never go bad. Their relationship with the D.I.A. and other government agencies they will have to deal with is not so cut and dried for both of these people have had good reason to not trust too deeply.