In the recent Justice League story, The Darkseid War issue #42, it was revealed that Batman knew the Joker's real identity. Now we the readers did not know (yet) but maybe in future editions. Batman sat on the knowledge of Mobius Chair and asked the question that has been haunting him.
Batman knows Joker's true identity.
Now for me, I'm not at ease with this. I am very much a man who loves to keep the mystery going. Having knowing the Jokers real name really takes away the added appeal of the Joker. It keeps people guessing all the time.
This is why he is by far one of the greatest villians in comic book history. Not only for his character which is sociopathic beyond compression. But also the close relationship between him and Batman. That relationship is the nucleus of the appeal of the Joker. Batman not knowing who the Joker is and vice versa.
You look at the other villians in the DC universe. Penguin, Riddler, Killer Croc, Deathstroke, Lex Luther, Metallo, Clayface etc. All are great villians but we know their identity which dilutes their character a bit. We stop caring for these characters because we know everything about them....basically the mystery is gone. However, with the Joker the appeal of his secrecy is paramount to his character. It's the attraction that draws us to him....the not knowing.
If we all discover his identity his character will be less appealing. Look at Wolverine or Snake Eyes. Both were in the same situation as the Joker but both were given origin stories which kind of ruined it for me.
The origin of Wolverine. A brilliant story but was it really worth telling?
Snake Eyes origins were told but was it a good move?
Keeping the mystery going is what people love to do. It keeps people coming back for more. You see all the people who look for Easter Eggs in movies and comics. Once they do, they discuss it and keep discussing it. The same goes for the Joker's real name. I see the Joker's real name as only The Joker!