The Joker is one of the most famous comic book villains in the world. The character was introduced in 1940, in the debut issue of Batman.
So technically, he has been around for as long as Batman.
Green hair, pale white face and a wide mouth grin are his signature characteristics. But over the decades, there have been minor changes here and there.
This is a cool graphic showing the evolution of The Joker, from 1940 to 2019.