The Joker is a psychopathic criminal from Gotham City. He is the mortal enemy of Batman. Unlike other criminals, who have some kind of goal in mind, the Joker merely wishes to have fun with his shenanigans.
In Downfall Parodies
The Joker portrayed in parodies is typically the Joker in The Dark Knight.
The Joker was also instrumental in Hitler's attempt to kill Justin Bieber. He answered Hitler's advertisement for a killer to destroy the singer, and Dolfy approved him for the job. The Joker chased Bieber throughout Berlin before finally gunning him down.
The Joker is also used as Soalric's main avatar, and his video signature features the Joker heavily.
VileHQ is known to have featured John Doe from Batman: The Telltale Series, who later becomes Telltale's incarnation of the Joker, in his Downfall Parodies and other Downfall-related content, likewise he has added Downfall content in his Batman Telltale YouTube Poops, which typically involve John. However this incarnation of the Joker seems to be on good terms with Hitler. He even took over as the Führer after Amanda Waller ended up sending Hitler to the hospital after a failed attempt to frame John for murder. However unlike most versions of the Joker, John seems to be more of a good guy.