J. Bartlett was a Starfleet officer assigned to the starship USS Voyager, a commander who was killed in the line of duty sometime before 2377. Bartlett's name was read out on a casualty report of all who died during that vessel's journey through the Delta Quadrant. (VOY: "Imperfection")
Judging by the commander rank, it is possible this name refers to one of the senior officers killed en route when Voyager was dragged to the Delta Quadrant, perhaps the unseen chief engineer. Then again, it might not. In any case, it seems odd that a commander would be assigned to Voyager, and outrank the executive officer, Lieutenant Commander Cavit. However, it is possible this was a staff officer, not in the chain of command, such as Commander Deanna Troi outranking second officer Lieutenant Commander Data on the USS Enterprise-D.
As with many names on the "Imperfection" list, J. Bartlett is an in-joke (albeit misspelled) referring to Martin Sheen's character on The West Wing. In that respect, he would indeed be a commander, as well as a chief.