Defense Commissioner
The Federation Defense Commissioner is the most senior civilian (under the President) within the Starfleet chain of command. He answers directly to the President and the Federation Council. The position sometimes fluctuates between career-long politicians and Starfleet veterans (oftentimes retired Commanders of Starfleet).
In 2208, upon Starfleet's re-organization, the position is re-titled as Secretary of Starfleet.
While the character is named and oftentimes a former PC, the Defense Commissioner is not a PC.