Warning: Wikipedia contains spoilers.
Dr. Zimmerman is the Director of Holographic imaging and Programming on Jupiter Station, and is regarded as the father of modern holography. He has won the Daystrom Prize for Holography on at least one occasion, and is best known publicly for creating the EMH program. Ironically, he regards the EMH series as his greatest personal failure.
When Zimmerman received the commission to build the EMH Mark I, he put far too of himself into the task at hand. While there was nothing wrong with the basic operation of the program, Zimmerman modelled it on himself, from physical parameters to psychological profile, and it was that physiological profile that proved the program's downfall.
Zimmerman himself can best be described as irritable, impatient, arrogant, stubborn and brilliant, all traits which he passed on to the EMH Mark I, leading Star Fleet Medical to reassign the series after receiving complaints of the program possessing an extremely poor bedside manner. Zimmerman attempted to have the programs decommissioned but was overruled at a higher level, leaving the Mark I series to be used for menial labour.
The failure of the Mark I turned Zimmerman into a recluse as he tried desperately to fix the EMH series. As of the episode 'Lifeline' he had not left Jupiter Station for four years, and was a far cry from the man who visited Deep Space Nine not long before in the episode 'Dr Bashir, I Presume?'
Towards the end of the sixth season of Voyager, Dr. Zimmerman developed an unknown terminal cellular degeneration disease Federation doctors could not find a cure for. When even his own latter generation EMH's could not find a cure, Zimmerman resigned himself to a slow and painful death. Ironically, the Doctor, sole surviving EMH Mark I, serving as Chief Medical Officer aboard the USS Voyager, was able to discover a cure and had himself transferred back to the Alpha Quadrant to administer it.
Faced with such a vivid reminder of his failure, Zimmerman refused the treatment for the bulk of a month and was openly derisive of the Doctor. It took the combined efforts of Reginald Barclay, who aided in the creation of the EMH; Deanna Troi, councillor aboard the USS Enterprise; and Haley, Zimmerman's long-standing personal assistant, to force a resolution between the two. Haley, it should be noted, is another advanced holographic program with more run time than the Doctor, and is something of a daughter figure to Zimmerman.
Dr. Lewis Zimmerman is played by Robert Picardo, who also plays the Doctor.