One of the big secrets to be discovered in the last season of the new Battlestar Galactica show is who the final Cylon is. We knew in the original miniseries that there were twelve models, and seven of those were gradually revealed over the course of the first two seasons. Then we saw no other models, and it became a mystery who the other five models were. Even the seven models didn't seem to know. Eventually one found out and got put in cold storage, and one of them really surprised her. She even apologized when she found out. But at the end of season three, four characters we had assumed to have been human all along turne out to be Cylons. The way they discovered it is that they had all been hearing the same music that no one else was hearing, and it had led them all to the same room. The producers have said these four really are Cylons, and yet they're different from the rest. The prophetic hybrid has said that they've been to earth, which must be how they all have within them the tune to a Bob Dylan song.
But what about the last model? We now know the model numbers of the first seven models we knew of. First we learned 2, 5, 6, and 8. In a recent episode this season we found out the others are 1, 3, and 4. That means the final five are 7 and 9-12. Wouldn't it make sense that the four we know about are 9-12 (as a set), and the still-missing one happens to be the symbolic number 7? It's unclear to me why Sharon would be higher than one of the five but lower than the rest, but perhaps that will be revealed, and perhaps her greater connection with the humans has something to do with it.
Here's my theory. Models 9-12 are a set. We now have seen models 1, 4, and 5 become a set separate from models 2, 6, and 8. I suspect something will happen with 3 and 7. But who might 7 be? I'm sure it's someone we've seen before, and I think it's likely that whoever it is was not on board the Galactica when the others began hearing the music, or we would have seen all five. That means it's probably someone on another ship or someone whose model we have encountered before is dead. It's probably a major enough character that it will be significant when we discover who it is, but it doesn't have to be a primary character. It could just be someone who wasn't on board the ship. Only one character stands out as important enough to be the final Cylon who wasn't on board. That's Tom Zarek. Wouldn't it be funny if the original Apollo turned out to have been a Cylon all along? The only other one not on board is Starbuck. The hints for it to be her would be overkill if she really is one, though, and these writers aren't that obvious. It's got to be deliberate misdirection.
But it might be someone who we've seen die. It could be Billy. I don't think he was on the show long enough for him to be likely, though. There's always the chance that it could be Admiral Cain. I don't think so, though, because I think they wanted her brutality to be oh-so-human. I doubt the other Pegasus characters would be important enough to get such a role, especially if it's the final one who number 3 was apologizing to when she discovered who they were. (Of course, they said things like that about Tigh and Tyrol, too, so this isn't a sure argument.) My guess is Ellen Tigh if it's someone dead, because we know she's still available for filming. She's already been in her husband's dream sequences this season, and he sees her when he sees a Cylon. So my guess is either Tom Zarek or Ellen Tigh, probably Ellen.
Of course, this is all undermined if the last five aren't a set and only the four we've seen. If that's so, then the fifth would be unrelated and thus might not have heard the music but have been there. Then it could be almost anyone.