DC Universe: After the Fall
"You want to know the only difference between you and me? One. Bad. Day."
Nothing more is known now about the Joker than was known when he first came to Gotham City almost 25 years ago, and the only thing that has changed since then is that the Joker is now, if anything, more brutal than he was then. The self-styled Clown Prince of Crime has no superpowers beyond a capacity for incredible violence and a skill at creating deadly mayhem.
The Joker was Batman‘s greatest enemy, and has come close to killing the Dark Knight more times than any other supervillain the world has ever known. However, the Joker drew much of his identity from his conflict with Batman, and it has long been speculated that he is the one person that the Joker could have never actually brought himself to truly kill. However, he was also adept at attacking the Dark Knight psychologically, and many hundreds – likely thousands – of lives have been lost in the crossfire of Joker’s relentless assaults on Batman’s soul.
However, when the Batman retired, the Joker lost that part of his identity, but he lacked the finality of knowing that Batman was dead. Instead he just vanished, without so much as a goodbye. This act of contempt whipped the Joker into a murderous frenzy, and since then has stepped up his actions in an attempt to draw his old nemesis out of hiding.
The Joker has had many skirmishes with the Red Hood, who he has only recently realized is the resurrected Jason Todd. Their latest clash cost the Joker his staunchest protege – Harley Quinn – whom the Red Hood beat to death with a crowbar before blowing up the warehouse she was hidden in just before the Joker himself could arrive. While the Joker took the loss laughing hysterically, the further increase in his brutality on civilians is telling that he may have been affected by her death more than he is letting on.