Final Five
The term "final five" collectively describes five of the twelve humanoid Cylon models whose identity, knowledge, or existence has been deliberately or accidentally lost to the known existing seven Cylon agent models. Currently, only seven of the twelve Cylon agent models are in use. Caprica-Six, when asked by Gaius Baltar about the five missing models, curtly replies that the other agents do not talk about the subject (Torn).
A Number Three copy, which Gaius Baltar calls by her human alias, D'Anna Biers, repeatedly commits suicide to get glimpses of five white-cloaked beings that she believes are images of the final five. She attempts to draw what she has seen, but has difficulty in doing so (Hero).
One sketch shows what looks like two men and three women. One is a man with dark hair and a widow's peak, one is a middle aged woman with shoulder-length hair, one appears to be a woman with long hair, one appears to be a man with short, possibly thinning light hair and one appears to a young woman with very short hair. If this interpretation is accurate, that would make six male Cylon models and six female Cylon models in total (The Passage).
Baltar's ability to see a virtual doppelganger of Caprica-Six (as well as enjoying her company in various pleasant environments) lead him to suggest that he might be using a Cylon technique known as projection. This suspicion, in turn, makes Baltar begin a personal inquiry into his own nature after allying himself with D'Anna-Three.
The two eventually make their way to the algae planet, where a Colonial structure, the fabled Temple of Five, awaits with possible answers ("The Passage", "The Eye of Jupiter"). When the system's dying star goes supernova a ray of light beams in through the temple. D'Anna steps into the light and suddenly finds herself back in the Opera House and before the five again. Upon seeing their faces she recognizes one of them in particular, saying she "couldn't have known." She is then pulled back and collapses in Baltar's arms, dying before she could tell him what she saw. The identity of the person she recognized remains a mystery (Rapture).