Ridley Fox and Nita Parris are agents of the CIA.
Interestingly neither are Americans by birth. Fox was a citizen of Canada while Parris originates from Barbados.
Fox was a soldier in Canada's Joint Task Force Two (JTF2), that nations equivalent of America's SEALS. He had been happy there and even happier with his engagement to Jessica. When she was killed, he nearly lost it completely. Drinking, fighting, not caring about anything except a desire to get even or get dead. After a particularly nasty altercation, he was likely headed to prison when General Downing from the CIA showed up with an offer. Work for him and the Agency and get a chance to strike back at the group that ordered the attack that took her life. Fox never had a moment of hesitation.
Parris is a black woman who, even after seven years had passed since she graduated Princeton, could still use the talent that made her a member of that university's track team. She did not consider herself fast so much as she thought everyone else was very slow. But running was not her only asset - her brain was exceptionally smart and she had earned a doctorate in biochemistry. The CIA leaped at the chance to recruit her.
Parris "never knew her parents. Her mother had passed away a few days after she was born, and her father was a ghost. Aunt Pauline, with the exception of a few other aunts, uncles, and cousins who lived in Barbados and England, was the only family that she knew." Being alone came in handy when it was necessary to go undercover or disappear for long periods on a mission. No one to miss her meant no one to lie to.
As the series begins, the two are not teammates - they sort of knew each other before but that did not work out so well. That would change soon, though, as both were after the same group just coming about it from different directions. When they met, it was hardly a match made in heaven but survival is powerful motive for cooperation. Partners is a loose term with these two, however, as they still operate on their own as well as together. Luckily for each of them, they can go it alone or play nice with others.