Jack Napier is one version of the Joker's real name. It is also the most widely excepted one.
House Of Cards
Jack of Spades
This time the Jack isn't me. Nah… It's dear ol' Jack Napier. That sap who thought the world was something more than it is. The world is nothing but a chaotic ball of dirt spinning through the cosmos on a crash course. But Jacky realizes that now, I've smartened him up. Too bad Jacky didn't make it much father than Jeannie. He's about as dead as you can be now. That's why he's a Spade, cause I buried him with one. Deep down in this handsome head of mine. Poor Jack should've known better than to listen to the voice in his head that only he could hear.