Nicholas Flamel, who is already known to change identities frequently, is the obvious candidate.

(And Flamel could also be Quirrell; Of the canon characters, there are four people likely to be as powerful as MoR Quirrell is: Dumbledore, Grindelwald, Voldemort, and Flamel. He's not Dumbledore; Grindelwald is probably still in Nurmengard; Voldemort is a distinct possibility, but that-one-infamous-post-we-all-know-about is, in my opinion, more likely to be a red herring than truth; Flamel is most likely. If Quirrell were not Flamel, Harry would be correct in assuming that Quirrell would kidnap Flamel if the stone were genuine. Lack of access to the elixir of life would also explain Quirrell's illness and the accelerated aging that Harry observed when Quirrell was in the infirmary. It also explains why Flamel knows that the Stone is hidden in Hogwarts. And if Quirrel/Flamel is female, that would explain how an attacker managed to intercept Hermione in the girls-only staircase. This theory also explains Eliezer's hints that a future story development will make it obvious that he is in no way shortchanging the female gender.)

I suspect that, if Flamel is Quirrell, the presence of the stone at Hogwarts is some elaborate trap to draw out Voldemort, whom Flamel may believe resides in, or is, Harry.

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