Ramona the Happy Hooker
Not a 'failed character' as such, as Ramona lives and works on Sesame Street, but is rarely seen by other characters due to keeping hours that are not 'child-friendly' - a term that sums Ramona up pretty well. Producers of the show asked Ramona to refrain from plying her trade during daylight hours after a live taping of the show went awry, ending in a near-PR disaster for the show and a $10,000 fine for the show's star, Big Bird. Mr. Bird is currently on hiatus.
Although unseen, Ramona is occasionally mentioned by other characters on the show - her daughter, Abby Cadabra, who lives with her in a flophouse at the end of the street - and her pimp, Count von Count.