"All it takes is one bad day to push the sanest man alive to lunacy." - Joker, The Killing Joke
The Joker is a character from the DC Universe. More particularly, he is the archenemy of Batman. Despite his name, he is not to be taken lightly, commonly referred to one of the most dangerous criminals in that continuum.
For more information about his many incarnations in comics and other media, see the Batman wiki.
In Badfic Edit
The Joker has been portrayed a variety of ways in the various media over the years. He's been a relatively harmless criminal, a genocidal psychopath or even just an off-kilter fellow who could be goaded into being a hero. Any agents dealing with the Joker in a fic should been well versed on which continuum he's from in order to accurately determine his OOC readings. As a general rule, though, if the Joker is too empathic to anyone except maybe his partner Harley Quinn or isn't having a cruel laugh at his victim's expense, he's probably OOC.
In the Batman film The Dark Knight, he is portrayed by Heath Ledger, which has resulted in many Mary Sues taking this sociopath in their arms, claiming he was just misunderstood and turning him into a woobie.
The Joker and the PPC Edit
The PPC has handled at least one Suefic involving the Nolanverse Joker: