The character is an alter ego of Frank Reynolds. He was introduced in episode "The D.E.N.N.I.S System" and was created by Frank to capitalize on the failed relationships of Dennis and Mac. He was the only member of the gang who was ever known as a doctor, and he had a brief stint as a pilot for a commercial airline. He also starred in the movie "The Gang Beats Boggs" where he gave another passenger a lethal dose of sleeping pills while they were on a flight to break Wade Boggs' record for beer drinking.
