The Joker

Speculation
The past of The Joker has been debated for years. The problem lies in the many different truths and half-truths told by the man himself over the years. Whether his life of crime started before or after becoming The Joker, is unknown. Who he was before The Joker, is unknown. The most popular theories involved him using the alias Jack Napier for a time, though this is unconfirmed. Another is that, he was part of the Red Hood robberies in 2009, this is also unconfirmed.

Batman Death Waltz
The Joker is currently being housed in Arkham Asylum after being apprehended by Batman. He started to be treated by Dr. Harleen Quinzel, his experience with her has been different from all other doctors in the past. She has broken rules for him and has started to get too close...

Harley Quinn
Joker broke out of Arkham with a planned attack from his outside thugs and corrupt insiders. He used this chance to torture Harleen Quinzel before escaping. The Joker had assumed that would be the last he'd seen of her. Instead though the doctor kept appearing before him, begging for a chance. Joker kept rejecting her but he also kept sparing her life. Until he offered her a chance to prove her love in some way... And she did by releasing a majority of the 'super-criminals' in Arkham Asylum. At this point, Joker finally made the decision to keep her, christening her Harley Quinn.