So who could it be? We've rounded up our three favorite theories.
1. Ali or her twin. In the book series Alison (Sasha Pieterse) had a twin, and there have been hints that the same might be true on the show. If so, Ali might still be alive, with her twin dead in her place — or vica-versa. Either way, we wouldn't put it beyond this mean girl to take her manipulations to the next level.
OK, yes. Ali was being stalked by "A", which makes this theory seem less seniscal, particularly if she doesn't have a twin on the show. But who's to say she didn't track down "A" and take over? Girl is scary.
2. CeCe. We don't trust CeCe. Could she be the head of everything? She seems smart enough, but what's her motivation?
3. Jenna. Who says the figure in red has to be a blonde, as opposed to someone wearing a wig? Jenna (Tammin Sursok) still has more motivation than almost anyone to torture the Liars, and with her MIA status this season, there's no way to know what she's up to. (On the other hand, we know what she was wearing to the masquerade ball, and it wasn't red and a blonde wig.)
Of course, these are hardly the only theories. Who do you think it is? Share your thoughts in the comments below.