The Joker is archenemy of Batman and Gotham's most wanted criminal.
The Joker (who's real name remains unknown) was a clown who was fired from his job and driven into denial. Unemployed, he overheard some guys talking about a crime they're going to do. He said he wanted to go with them. They made him wear a red hood (as or the name of the group) so people could think he was their leader. While robbing a chemical factory, Batman showed up. While confronting the unnamed man, he fell backwards into a bat of toxic waste causing his skin to turn white, his hair to turn green, his lips to turn red, and him to go mentally unstable.