The "Final Five" Cylons drawn in a crude sketch by Number Three
In the episode "Torn", Gaius Baltar questions Number Six as to why only seven humanoid models appeared on New Caprica and why he has seen only these same seven aboard the Basestar when she had told him previously that there were twelve models. Six replies that the Cylons do not talk about the other five models. Baltar has become worried that he may in fact be a Cylon himself and presses her on whether she would recognize one of the "Final Five", as he names them. Their conversation ends abruptly, leaving the truth of the other models unknown.
In the episode "Hero", D'Anna Biers has a dream immediately after she dies, where she sees five glowing figures. Later, in the episode "The Passage", she tries to hold on to the memory of their faces and draws pictures of them soon after downloading, but the memories fade and the drawings are too surrealistic to be of practical use. It is clear that she does not consciously know what the Final Five look like. In the episode "Rapture", D'Anna and Baltar reach the Eye of Jupiter and D'Anna receives a vision of the Final Five. She seems to recognize one of them, but the vision is cut short before the viewers see any detail of the figures. The shock kills her and upon awakening after downloading, she mentions her experience to Cavil. He confirms that the seven known Cylon models do not know who the Final Five are and that the pursuit of that information is strictly forbidden by their programming. As punishment for this and for other acts, Cavil then terminates her and her entire model is "boxed", taking the secret of the Final Five with her until she is unboxed.
In "Crossroads", four of the final five Cylons are revealed to themselves, each other, and the audience, though not to the human characters. Samuel Anders (Michael Trucco), Galen Tyrol (Aaron Douglas), Tory Foster (Rekha Sharma), and Saul Tigh (Michael Hogan) begin hearing the same hallucinatory music that no one else can perceive (a version of "All Along the Watchtower"). When the music draws them towards each other, a "switch goes off" in their minds and they realize that they are all Cylons. They resolve, nevertheless, to continue doing their duty as part of the fleet. The identity of the fifth and final Cylon remains unknown even to them. Their "awakening" seems to affect other Cylons as well, as in the episode "He That Believeth In Me", a Cylon Raider scans Anders then disengages the attack; in the same episode, aboard Galactica, Caprica-Six tells Laura Roslin that she can feel the Final Five and that they are near. It is later revealed in "Six of One" that the Twos, Sixes and Eights believe that the Raiders called off the attack because they sensed the Final Five, something the Ones, Fours and Fives disagree with. Cavil further claims that their original programmers programmed the seven Cylon models not to seek that information out. This disagreement leads directly to the Cylon Civil War, in which the Twos, Sixes, and Eights become known as the rebel faction. In "Revelations", an unboxed D'Anna reveals that only four of the Final Five are within the Colonial Fleet, and confirms that Tigh, Tyrol, Anders, and Foster are those four Cylons. A standoff between Galactica and the rebel Baseship results in the four being exposed as Cylons to the rest of the human Fleet, but Lee Adama, in his capacity as acting President, grants them amnesty as part of an alliance between the Fleet and the rebel Cylons.
In "Sometimes a Great Notion", it is revealed that all of the Final Five had previous lives on Earth, 2,000 years in the past, as part of the Thirteenth Tribe. Cylon testing procedures used on the tribe's remains reveal that they were all humanoid Cylons. As each of the Final Four explores the ruins, each begins to regain small memory fragments of their past life. It is when Saul Tigh remembers a snippet of his previous life that he discovers that his late wife, Ellen Tigh (Kate Vernon), must be the fifth and final Cylon, because she was present on Earth 2,000 years ago as well. During Saul's flashback, his wife insinuates that she had foreknowledge of the destruction of Earth when she states that plans have been made for her and Saul to be reborn.
In "No Exit" Ellen Tigh is revealed to have resurrected onboard one of the Resurrection Ships in the Cylon fleet after her poisoning, and to have subsequently been held prisoner by Cavil on a Basestar for 18 months. The past of the Final Five is also revealed: they lived on Earth 2,000 years ago and were researchers in rebuilding Cylon resurrection technology, which had been abandoned when the Thirteenth Tribe gained the ability to procreate. Saul and Ellen were married even back then, while Tyrol and Foster were in love and had plans to get married. The five learned of the upcoming holocaust from beings only they could see and worked to rebuild Resurrection. They succeeded and placed it on a ship, which they left in orbit of Earth. When the nuclear attack happened and they were killed (according to the series' writers, the cataclysm was an uprising of Earth's own Centurions that apparently destroyed both sides), they were reborn in new bodies on that ship in orbit. The Five then headed for the Colonies to warn them about making artificial life, but the trip took thousands of years due to there being no FTL drive on their ship. Due to traveling at near light speed, time dilation made it seem like less than that to them. They arrived during the First Cylon War, unbeknownst to the humans, and made a deal with the Centurions: stop the war and they would help them build human bodies. Thus the Final Five ended the First Cylon War. They built Number One, Cavil, and he helped them build seven other models, but after Ellen began to favour Number Seven, Cavil permanently destroyed the Number Seven line by contaminating the production. Ultimately Cavil locked the Final Five in a compartment and evacuated the oxygen, killing them. He then boxed them and apparently removed the memory of their identities from the other Cylons. He later unboxed and resurrected them, but first blocked their true memories and implanted false ones. Cavil periodically seeded them among the human populations starting with Saul, and then Ellen, to show them how bad humanity supposedly was. This shows that Cavil's claim that not seeking out the Final Five was a directive from the Cylons' original programmers was a lie, and that he was only pretending not to be aware of their identities himself. Ironically, the "Final Five" are in fact the first five humanoid Cylons to encounter the Colonials and the new Cylon Centurions after the Thirteenth Tribe's diaspora. Of the Five, only Anders and Ellen regain their true memories: when Anders gets shot in the head, it seems to break Cavil's block on his memory and Ellen's resurrection in a new body restores all of her memories. It is also revealed in The Plan that the Cavils planned for the Five to die in the destruction of the Colonies, download, regain their true memories and apologize for their faith in humanity. Despite the Cavils' hopes, the Final Five survived the destruction of the Colonies without resurrecting (which the Cavils hadn't believed possible) and Cavil even keeps Ellen alive so she can suffer more and learn her "lesson". The Cavils' hopes that the Five learn a "lesson" is shown not to have worked, as all of the Five (except Tory Foster) maintain their loyalty to humanity even after the truth comes out.
Similar to their role in ending the First Cylon War, the Final Five have a major role in ending the second war. During the Battle of The Colony, the Five take a prominent role in the battle, especially Anders who, acting as Galactica's Hybrid, shuts down the Colony's weapons and Hybrids. The Five also nearly bring a peace between human and Cylon by exchanging Resurrection technology (which can only be rebuilt by the Five combining their knowledge together) for peace. However, a side-effect of the connection they make is they see each other's memories, and Tyrol learns that Foster murdered his wife. In response, he strangles her, and the temporary peace is shattered. Later, Anders flies the fleet into Earth's sun, destroying the fleet and himself, leaving only three of the Final Five alive. As the Resurrection Hub had been destroyed, their deaths are permanent
Ronald D. Moore confirmed in an interview that the four are indeed fully Cylon — although "fundamentally different" from the others. Moore has also stated that unlike the other Humanoid Cylon models, the Final Five do not have model numbers. This is because in their original lives they were born to Cylon parents through sexual reproduction, rather than being built, as stated in "No Exit".