Nick Riviera is a quack doctor, constantly appearing on infomercials promising all sorts of bizarre medical offers and turning his operations into a TV spectacle. Riviera has a quasi Eastern-european accent, a medical degree from a community college at which he apparently spent much of his time prescribing "anything I want" to a succession of attractive women back in the 1970s, and a great deal of luck. Dr. Riviera also studied at the Hollywood School of Medicine.
Riviera is notable for his exclamation upon entering a room of "Hi, everybody!" which is followed by a response of "Hi, Dr. Nick!" from the other characters.
He has operated on the Simpson family a couple of times, notably when Homer could not afford a heart bypass from his regular doctor. Lisa attended the live audience(!) for the operation and saved the day by guiding the obviously clueless Dr. Riviera through the operation.
He is also shown as an inventor/huckster (in the style of Ron Popeil) on "I can't believe they invented it".