The upcoming Joker origin film that will be directed by Todd Phillips and star Joaquin Phoenix has revealed the "real name" of the man who would become Joker. The name that they are using is Arthur Fleck. This is notable, of course, as the Joker, generally speaking is famous for how he does not have a "real" name. Heck, the comic books have made it clear to avoid giving Joker an "official" origin at all.
However, Arthur Fleck is not the first time that somebody came up with a "real name" for the Joker. This would be the name that, until now, had been the only other "official name" ever associated with the Joker. That name is "Jack Napier."
The reason behind the creation of the Jack Napier name is because of how Tim Burton decided to handle the Joker's origin in 1989's Batman. They went with an actual defined origin for the villain. He was a mobster who was having an affair with the girlfriend of the head of the mob, Carl Grissom. Therefore, if you are going to have a guy be a well-established member of the mob, he can't very well not have a name. So Burton came up with Jack Napier, which was a dual tribute -- Jack for Jack Nicholson, who played the Joker in the film and Napier for Alan Napier, who played Alfred Pennyworth in the 1960s Batman TV series.
Grissom learned of the affair and conspired to have Napier assassinated at a job at ACME Chemicals. Instead of dying, though, Napier instead falls into a vat of chemicals and is transformed into the Joker. He gets his revenge on Grissom and almost poisons all of Gotham City before being dying in battle with Batman.
As Napier dies, we discover that it was actually a young Napier that killed Bruce Wayne's parents in that fateful alley in Gotham when Bruce was a boy...
The film was a smash success, so the name Jack Napier was now forever associated with the Joker, but what is interesting is that the comic books still stayed away from giving the Joker any sort of real name. Interestingly, the first possible origin for the Joker was given just a year before Batman was released, with Alan Moore and Brian Bolland's Batman: The Killing Joke, which suggested a possible origin for the Joker was that he was a failed stand-up comedian who agreed to play the role of a costumed villain, the Red Hood, for a score that went horribly wrong. The idea is that one bad day could conceivably transform a person's entire life and turn a normal person into a villain. The Red Hood identity, by the way, was the only aspect of Joker's past that had ever been confirmed before. It was revealed in a 1951 Detective Comics story that the Joker had, at one time, gone by the identity of the Red Hood.