Scylla is an agent with the Division.
The Division is an American clandestine agency with assassination as one of its main specialties. If a matter requires special handling, up to and including elimination, it is the agents of this group that gets the call. Scylla is one of the oldest and best agents this department has had.
And yet while the character is the key protagonist of the second book in the series, and plays a major role in the first, this agent is not the man first thought of by anyone who has read the books or, more commonly, has seen the movie version of the first book. Star billing in that book, Marathon Man, easily goes first to the younger brother of Scylla, a graduate student named Thomas Levy but known more by his nickname of "Babe". It is Babe's misfortune to be related to the agent and even more unfortunate that he is suspected by a desperate Nazi war criminal of having information he doesn't have.
It is the stalking and kidnapping of Babe by the criminal that made the first book and movie so captivating and chilling - that and fantastic acting by Dustin Hoffman and even better role-playing by the incomparable Lawrence Olivier. In that movie, the role of Scylla was superbly handled by Roy Scheider.
In the second book, it is Scylla who is the star and he shows just how good he can be. Many think him too old to still be doing what he does and the question of his injury that nearly killed him is a factor but Scylla shows that he is still a man to be depended upon by his bosses and feared by his targets. And still loved and admired by his younger brother.