A major addition to the character was the introduction of the character Harley Quinn.
Originally introduced in Batman: The Animated Series, Quinn is a clinical psychiatrist who falls hopelessly in love with the Joker in Arkham Asylum after he relays his tale of having an abusive father and a runaway mother, and now serves as his loyal, if daffy, sidekick, costumed in a skintight harlequin suit.
Wizard magazine listed him the #1 villain of all time in 2006.
Interpretations of the Joker's appearance in other media include Cesar Romero's in the 1960s Batman television series, Jack Nicholson's in Tim Burton's Batman, and Mark Hamill's in Batman: The Animated Series, other DC Animated Universe shows and the Batman: Arkham games.
When I saw what a black, awful joke the world was, I went crazy as a coot! Do you know how many times we've come close to World War III over a flock of geese on a computer screen? Anything anybody ever valued or struggled for, it's all just a monstrous, demented gag. He was first introduced in Batman #1 (Spring 1940) and has remained consistently popular.