Emergency medical hologram
A typical Starfleet EMH has technical proficiency in many fields of medicine, but they tend to have a poor bedside manner, not having been designed to deal with patients on a regular basis for routine care.
When the chief medical officer of the USS Voyager was killed, the ship's EMH was forced to serve as CMO for the ship. Consequently, this medical hologram spent far more time actively running than any other EMH in Starfleet. Since the EMH program is an artificial intelligence with learning ability, the Voyager EMH developed a far more complex personality than most EMH programs. Voyager's EMH is probably the only truly likable personality on Voyager.
"Seems I've found myself on the voyage of the damned."