Nick had a relationship with Charleston prior to Kabul.
Nick was abducted from Charleston's car at the end of Bang, Bang as the pair were getting intimate.
Nick was contracted out by the CIA to interrogate and turn Omar Abdul Fatah. He was one of five on board the level ten black ship, the Midnight City where Nick employed tactics including physical violence to try and turn Fatah. It is suggested that Nick has done prior work with the CIA perhaps when he states to Charlie that "I'm not CIA on this one".