Deputy Defense Commissioner
The Deputy Defense Commissioner is the second senior civilian (under the President) within the Starfleet chain of command. He works directly for the Defense Commissioner and assumes some of the duties of that office, as assigned. He is also slated to perform as the Acting Defense Commissioner in the event the actual Commissioner is unavailable. The position is filled by either career-long politicians or Starfleet veterans, and usually the opposite of whatever the standing Commissioner is.
In 2208, upon Starfleet's re-organization, the position is re-titled as Deputy Secretary of Starfleet.
While the character is named and oftentimes a former PC, the Secretary of Starfleet is not a PC.