One of the two enemies exclusive to Chicago: Stealth.
A C.I.A. agent.
These men roam Chicago aimlessly, until they encounter you, that is. If unarmed, they will treat you as if you were a normal person, saying things like: "Greetings citizen!" However, punch them or pull out a weapon, and the agents will run away. They are a curious foil to the FBI agents as they are the opposite to them almost every way. As such, killing these fools fails the mission. This can be incredibly difficult, as they seek refuge among the other gaurds. They do not carry any weapons, and are very pathetic.
Also see: FBI Agent