What we know about the four revealed Cylons of the Final Five:
Tyrol – a mechanic crew chief aboard a battlestar. Clearly the best mechanic on the show, and by implication aboard the fleet. Not unreasonable to assume one of the best mechanics in the Colonies overall.
Tori – a political aide. Managed to become aide to the most powerful politician and important civilian leader aboard the fleet.
Anders – former sports superstar turned resistance fighter turned viper pilot. Managed to become the Michael Jordan of his sport, then became a leader in the Caprica resistance, and then a leader in the New Caprica resistance. Now a viper pilot, which is the elite fighter jock corps of the Colonial military.
Tigh – war hero during the Cylon War, best XO in the fleet, most self-destructive alcoholic in the fleet (these latter two things not simultaneously. Call it an Exclusive-OR). Also rose to top leader of resistance on New Caprica.
and, most importantly: all of them managed to 1. survive the Colonial holocaust, 2. get aboard Galactica (even Anders who was left behind), and 3. continued their upwards career trajectories despite fierce competition.
do we see a pattern? clearly, they are overachievers even by the already rarefied standards of the Galactica crew (and the demands of narrative focus). Presumably, the Fifth won’t be a slacker in whatever it is that they do.
There are other major clues, the best being the infamous “Last Supper” photo (click to enlarge):
… which has an empty place setting for the Fifth (and thus implying, with Moore confirming, that none of the other characters pictured are the Fifth, ruling out Baltar, Adama, Roslin, Helo, Apollo, and Starbuck). The Holy Grail is a nice touch, implying that the Fifth has an intimate connection to Earth.
The other major inference we can make is that the Fifth probably already knows that they are a Cylon and has known all along. This is because they did not respond to the Activation song which drew the other Four together. Therefore, the Fifth must be somewhat aloof and a loner, as they are acting to their own agenda and cannot afford personal scrutiny.
I am assuming that the Fifth is aboard Galactica and not aboard some other ship in the fleet, because that’s where all the major characters usually are (and the show is named after her). I suppose it is possible that the Fifth is some random dude aboard the Garbage Schooner or something but this seems unlikely. The Galactica is the nexus of the fleet, and given the likely overachiever status of the Fifth it seems improbable that they’d gravitate anywhere else.
So, who is aboard Galactica, is a major character, an overachiever, has been a bit of a loner and aloof, and is absent from the Last Supper photo?
I think only one character fits:
My guess is Felix Gaeta.