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.
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).
Previously, 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).
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 them ("The Passage," "The Eye of Jupiter").