The Joker is arguably Batman's greatest foe, his arch-enemy and opposite number - the chaos to his order. And believe it or not, it's been that way almost since the first time Joker appeared in 1940's Batman #1.
Created by Bob Kane, Jerry Robinson, and Bill Finger, the Joker was Batman's first official supervillain (though he had previously fought the Mad Monk, a vampire who wasn't considered a traditional supervillain at the time). Interestingly though, the Joker wasn't intended to stick around past that first appearance, seemingly dying in the initial version of the story.
