Najix
Najix is an alien who greets the boys after they are abducted in the episode Cancelled. Najix has the ability to change his appearance - which is a relief given the grotesque nature of his actual appearance. Najix's most well known appearance, a taco that craps ice cream, was selected by Eric Cartman.
Najix also appears to be in charge of the reality TV show Earth and the Visitors who help him run it.