The EMH Mark II was the upgraded medical hologram of USS Prometheus, who teamed up with Voyager's Doctor to retake the ship from Romulans.
The EMH Mark II was a newer version of the Emergency Medical Hologram installed aboard USS Prometheus, an experimental Starfleet ship with a multi-vector assault mode. Considering himself more advanced than the original, he was arrogant and quick to dismiss his predecessor. Andy Dick played him.
In Message in a Bottle, Voyager's Doctor was sent through an alien relay network to the Alpha Quadrant and found Prometheus seized by Romulans. The two holograms bickered constantly but managed to gas the Romulans and fly the ship until Starfleet arrived, and the Doctor learned of the Dominion War and that Starfleet now knew Voyager was alive.