The Joker is a fictional character, a comic book supervillain published by DC Comics. He is the archenemy of Batman, having been directly responsible for numerous tragedies in Batman's life, including the paralysis of Barbara Gordon and the death of Jason Todd, the second Robin. Created by Jerry Robinson, Bill Finger and Bob Kane, the character first appeared in Batman #1.
In "Don't Blow Your Wadska", Wadska has a picture of himself dressed and posing with the Joker as imagined in the the film The Dark Knight on his wall of heroes.