Joker

Joker, once known as Jack, is the arch-nemesis of Batman.

Biography
Appearing to be the most evil, deranged, and flat out psychotic killer of all comic book villains, The Joker is the primary antagonist of Batman. He is the complete opposite of Batman, in everything from appearance (Batman tends to be a rather large and muscular man, while The Joker is typically a man of average height or even taller possessing a skinny frame), to ideals. The Joker views murder as sport, and will not hesitate to toy with his food before he eats it, even if said food are the men he has hired to work for him. As intimidating as he is psychotic, he still manages to recruit hordes of henchmen despite his maniacal insanity.

Batman: The Adventure Continues

 * No Laughing Matter (first apperance)
 * Hush
 * Girls Night Out
 * Knightfall
 * Arkham City

Batman Beyond Unlimited

 * Son of the Bat (mentioned only)
 * So Funny I Forgot to Laugh
 * The Jokerz Strike Back
 * Legacy (flashback from Return of the Joker only)
 * Calling All Villains (cameo in Arkham Asylum)

Justice League Heroes

 * Why So Serious?
 * Hush Little Baby (mentioned only)
 * Final Countdown, Part One (cameo)
 * Final Countdown, Part Two (final apperance)