Dr. William Janus made his first and so far only appearance in Season Fifteen episode "City Sushi". Dr. Janus is the split personality of Tuong Lu Kim, proprietor of the City Wok Chinese restaurant.
Background
Dr. William Janus becomes Butters Stotch's therapist after he inadvertently starts an Asian Turf War between Junichi Takiyama owner of City Sushi and Tuong Lu Kim's City Wok. The doctor sends Butters on a mission to investigate "Billy", one of Janus' other personas, and discovers Tuong Lu Kim is and always has been yet another split-personality of the therapist.