Jim Dennison

Jim Dennison was a Starfleet officer in the early 25th century. He commanded the Federation starship USS Firestorm (NCC-144017-F), as of late-2410. 

2409
In 2409, Jim served as Admiral Nat's first officer and starship captain during special task forces against the Borg and Undine, presumably being involved with multiple missions of temporal nature that the admiral went on. (Page 2)

2410
In 2410, Jim assisted Admiral Nat in various operations for the Federation Starfleet.

Changeling Infiltration
When Nat had been secretly replaced by a changeling, Jim was one to side with the admiral. Jim was eventually allowed back in Starfleet, of course, when it was discovered that he, nor anyone of the admiral's officers, had known of the admiral's true intentions.

The changeling was eventually realized when the alternate universe Sarah Walker was revealed to be a changeling herself, and the admiral's crew eventually found the real Admiral Nat in a Tholian facility in the Azure Nebula, after which the admiral resumed his duties and commanded operations against his mirror counterpart. Jim then resumed his previous position.

When Admiral Nat decided to permanently transfer his flag to the USS Sally (NCC-201591), Jim initially served as his first officer. When repairs to the USS Firestorm were completed, he was given command of the vessel in question, leading a mission to upload a specially designed virus to liberate a large number of Borg from a Borg unimatrix, succeeding, although it was eventually discovered that the Firestorm's construction had been rushed and that it's newfound lack of Borg auto-repair systems had left it unfit for combat duty.

Alternate Universes
There are, as of 2410, only one alternate universe version of Jim is active; Sean Dennison.

Trivia
Since the mirror of version of Nat had his officers go by their middle names, it is reasonable to believe that Jim's middle name is Sean, however, this has not been confirmed in the prime universe.