By this, I mean someone famous enough to have a Wikipedia article written about him/her. My first thought was Banksy, and Wiki says there are reports of names, but they conflict, and there doesn’t seem to be a consensus.
Is there anyone else who has managed to hide his/her real ID? Wasn’t there some singer in the early 80s who used voice filters to do interviews so that even the gender was in doubt? I remember seeing an interview, but I was so pop-culture ignorant at the time that I don’t know anything more.
I don’t know about “popular” but the Zodiac Killer has entries and the identity, sex, and current status (alive?) is unknown. Perhaps you mean celebrities?
I also believe the identity of the “Anti-Christ” is unknown but there are alot of speculation currently involving everyone from the President thru Dick Chenny to the Pope.
Deep Throat kept his identity secret for well over three decades – and it’s not like anyone figured it out, it’s that he simply decided to announce himself back in '05.
Jandek, a weird folk/new age/blues musician with a modest but fanatical cult following, is thought to be a certain Sterling Richard Smith, but this has never been confirmed. A very interesting character.
There is a guitar player named Buckethead (he plays with a KFC bucket over his head, with eye slits cut out of it) who is active in the jamband and heavy metal scenes (two musical genres that usually dont overlap much) who’s real identity is in question.
At one point he was supposedly Trey Anastasio’s (the guitarist of Phish) brother or even his alter-ego, (depending on who you talked to and how high they were) but I think that at one point they actually jammed onstage together at the Bonnaroo Music Festival, so that particular rumor faded away…
I don’t know if anyone has conclusively claimed to know what exactly the story is with this guy, but by only playing live with his face covered, he has garnered more attention that his mediocre (IMHO) music would otherwise merit.