The Joker's real name is unknown, though according to the first movie starring Nicholson, it might of been Jack Napier. The catalyst of his insanity has different tellings:
Originally a failed comedian, coerced into crime after the deaths of his wife and unborn child. Disguised as the "Red Hood", he failed to steal from Monarch Playing Cards. Running from Batman, he fell into a waste pool by the Ace Chemical Processing.
Surving the accident, it coloured his hair green, bleached his skin white and deformed his mouth into a red smile. Using eccentric weapons like acid spilling flowers and static joy-buzzers, he sadistically amuses his victims before he murders them. Other schemes include bribery and terroism.
The sidekick and former pyschiatrist of The Joker is Harley Quin (a.k.a Harleen Quinzel). She accompanies and supports her love interest in his crimes in Gotham City.