[Spoilers!] Dark Knight

[Yamu]

If you didn't catch it he tells a completely different story to Rachel about how he got his scars. He talks about his wife and chewing on a razor blade. Neither of them are true, he is a liar and a psychopath trying to freak people out with his horrible past, that is in fact not true. If you did not like the Joker because of his backstory you missed the point because there is not one, he tells two different versions and starts to tell a second one to Batman at the end but is stopped.

Nolan wrote the Joker to be a force of nature not a character. His story is not what is important, Batman's is. He comes out of nowhere and doesn't leave either, he is just there.