It's highly possible that he was a prison guard. His location, name, and the key he carries hint to that.
Martin has deciphered a second item needed for the ritual: the 'blood of a Divine'. He could not imagine a way to obtain such a thing, until Jauffre suggested a solution: the blood of Tiber Septim, who was once mortal but became a god. The Hero should ask Jauffre how to obtain this blood.