Joker

[close] His real name has never been revealed (aside for Jack Napier in the Tim Burton film, but this is not canon). His deformity was caused by wading home through a vat of chemicals while pulling a criminal job as the Red Hood. The Joker is utterly mad. In his very first appearance in Batman #1 (1940), he was not only stealing jewels but unleashing his "joker gas" on innocent people as part of his crimes. Eventually, he was executed, but he found a way for his men to revive him, which allowed him to walk free under double jeopardy laws. Subsequently, he toned down his act and added more humor along with a determined refusal to kill his greatest enemy, the Batman.

By the late 1980s, he was back to his sadistic self, plaguing most of the DC superheroes, especially the Batman. He shot Barbara Gordon, once Batgirl, and she became confined to a wheelchair. For many years she thus operates as Birds of Prey leader, Oracle, before physical therapy allowed her to once again become Batgirl.

In the alternate future of Kingdom Come, he even killed Lois Lane, and was then killed by Magog after being taken into police custody. When the public agreed with Magog, as opposed to Superman's desire that the Joker be given a fair trial, Superman retired.