The Joker

The Joker (Jack Napier) is a psychotic mass murderer who has over the years carved up a reputation in Gotham as the city's most dangerous supervillain and Batman's most deadly enemy.

Currently, he is locked away in Arkham Asylum for the Criminally Insane after his latest killing spree, but it's only a matter of time before he'll eventually break out.

Biography
Jack was a man under Carmine Falcone. He was one of the best men on the job, always carrying around his lucky deck. During a robbery at Axis Chemicals, Batman pushed him into a vat of toxic chemicals. He resulting damage to him caused him to go insane. He immediately kill3d Facone and became a crime boss in Gotham.