Sierra six
(Book)

"It's been more than a decade since the Gray Man's first mission, but the trouble's just getting started ... Before he was the Gray Man, Court Gentry was Sierra Six, the junior member of Zack Hightower's CIA Special Activities team, Golf Sierra, aka the Goon Squad. Court's skills were fearsome and undeniable. Too bad he wasn't also a team player. It took weeks of training and withering scrutiny to mold him into one, but the effort was worth it--on their first mission they took out a terrorist leader. The prize was invaluable; the cost was agonizing. Now, years have passed. The Gray Man is on a simple mission when he sees a ghost: the long-dead terrorist, though he's remarkably energetic for a dead man. A decade of time hasn't changed the Gray Man. Whatever the cost, he isn't one to leave a job unfinished or a blood debt unpaid"--