Jack Napier (Earth-1999)

The Joker is an incredibly dangerous psychopathic criminal who is a constant nemesis of Batman and Robin.

History
Before becoming the Joker, John Arthur "Jack" Napier was a former lab assistant and failed stand-up comedian with a wife named Jeannie. Jeannie was pregnant with their first child, but due to his difficult life he started becoming erratic and mentally imbalanced. Jeannie, fearing for both her life and the life of their unborn child, went to the police, who helped her fake her death and move to Alaska to escape her husband. The man who would become the Joker was informed that his wife and child had died, forcing him to experience "one bad day".

The Joker became Gotham's most sadistic and twisted crime boss and is insane, but also a genius, having knowledge of chemistry, physics, and engineering; Joker invents gag-based gadgetry whilst concocting criminal schemes. The Joker also cheats death frequently, having been thought killed several times only to come back and wreak havoc once more. He has many minions serving under him, most notably his self-proclaimed lover, Harley Quinn, his psychiatrist at the Arkham Asylum who fell in love with him there. The Joker is a mass murderer and has been known to kill many people, even his own henchmen.

Joker is widely accepted to be Batman's arch-enemy, which he is noted to be in the January/February 2012 issue of LEGO Magazine.