There are twelve models of human-form Cylons. Seven, all known among the Cylons themselves by number, were revealed to the humans in the course of their early conflicts after the Cylon attack on the Twelve Colonies. The "Final Five" were said to be fundamentally different, and the numbered models were programmed not to seek them out; but D'Anna, a Number Three, had visions in which she saw their identities. Four of the Final Five were revealed to each other at the end of season 3, and exposed to the humans and the Cylons in "Revelations" (4x10).
1. Dean Stockwell as Number One (Brother Cavil)
Cavil first appeared as a member of the ministry whose counseling Tyrol sought after a series of nightmares made him wonder if he was a Cylon. (Cavil responded, apparently joking, that Tyrol couldn't be a Cylon because he was one and he hadn't seen him at any of the meetings.) Cavil's distinctive capacity for dry humor does nothing to blunt his model's ruthlessness. During the occupation of New Caprica he argued for culling the human population; he put all of the Threes into storage in punishment for seeking the knowledge of the Final Five; and he sought to destroy the rebel Cylons led by the Sixes rather than allow the Cylons' destiny to be compromised.
2. Callum Keith Rennie as Number Two (Leoben Conoy)
Leoben is the most philosophical of the Cylons, and has a disconcerting ability to force one to question one's assumptions. Early on Commander Adama was forced to spend time in Leoben's company and he became especially leery of this model and his inquisitive nature. Leoben has an obsession with Kara Thrace, sensing at some level her central role in the future of both their races, and has appeared on several occasions to challenge and guide her. While on occasion these encounters have been benevolent, she has not forgotten his imprisonment of her during the occupation, during which he forced her to live with him as a couple and attempted to gain her love, despite her constant (successful) efforts to kill him.
3. Lucy Lawless as Number Three (D'Anna Biers)
D'Anna first appeared as a documentary filmmaker who convinced Commander Adama to allow her to shoot a piece on the pilots officers aboard Galactica. D'Anna increasingly sought understanding of the destiny of the Cylons, including the hidden Final Five. Finally in a vision the identities of the Final Five were laid bare to her, but in punishment all the Threes were "boxed" (put in storage). The rebel Cylons, led by the Sixes, became convinced that the Final Five would know the way to Earth and mounted a rescue of the Threes which would end in destruction of the Resurrection Hub, permanently ending the Cylons' ability to resurrect. One Three, D'Anna, survived, and confirmed the identities of the four revealed as being part of the Five.
4. Rick Worthy as Number Four (Simon)
Number Four is primarily a medic. He appears most prominently during the series as the doctor at a farm on Caprica where Kara Thrace is experimented upon, as part of his interest in the possibilities of reproduction for the humanoid Cylons. In "The Plan," it turns out there was a Number Four in the fleet who had married a human and was step-father to her child; and there was another Number Four among the rebels led by Sam Anders on occupied Caprica.
5. Matthew Bennett as Number Five (Aaron Doral)
Number Five is first seen as a marketing and public relations expert, who's on board the Galactica in connection with its conversion into a museum at the time of the Cylon attack. Later, another Number Five acts as a suicide bomber on the Galactica; this is the event that exposes the humanoid Cylons to the general population of human survivors. Generally speaking, the Fives are among the models that are less sympathetic toward the humans.
6. Tricia Helfer as Number Six
Number Six, a statuesque beauty, has had a pivotal role in the development of the Cylons. One of the Sixes, later known as Caprica Six, seduced Gaius Baltar and so gained access to the human defense grid. But she later came to question the relentless war between humans and Cylons. Seeking a new future for the Cylons by overcoming their self-assessment as machines, she created a dissent movement, particularly among Sixes and Eights, that led ultimately to a Cylon civil war against the Ones and oth
7. Number Seven (Daniel)An artistic model to whose closeness to the Final Five creator-Cylons caused Number One to sabotage his production, essentially aborting his existence as an active model, Number Seven does not exist during the Cylon wars and plays no role in the conflict with the humans (other than as part of Number One's motivations in relation to the Final Five). He's never seen on screen during the run of 'Battlestar Galactica,' but his name connects him to Daniel Graystone, a central character in 'Caprica.'
8. Grace Park as Number Eight (Boomer, Sharon "Athena" Agathon)
A fighter pilot aboard Galactica, Boomer realized she might be a sleeper Cylon agent when she woke one day soaking wet and holding an explosive, just before the ship's water tanks exploded. Exposed as a Cylon when another Eight, the one that had fallen in love with Helo on Caprica, returned to Galactica. Boomer was assassinated and cried out in anguish when she found herself resurrected, not wanting to believe she was a Cylon. Meanwhile the other Sharon slowly gained the trust of Commander Adama and was eventually allowed to join the ranks as a pilot, gaining the nickname Athena. Among the Cylons, the Eights sided with the Sixes in the civil war, but Boomer, feeling alienated from her model, took Cavil's side.
9. Michael Trucco as Samuel Anders
Sam was an athlete who became a resistance leader among a handful of humans who avoided being killed by the Cylons occupying Caprica. Encountered by Kara during a secret mission, Sam later returned with her to Galactica with a small group of survivors. They married on New Caprica, creating new tension between Kara and Lee. After the liberation Kara and Sam had a stormy relationship, but Sam remained loyal to Kara and sought to help in the struggle to defend humanity by joining up and training to become a pilot. Since finding out he was a Cylon Sam remained the most level-headed of the four, urging calm as the others reacted emotionally to their plight -- though Sam feared what would happen when Kara found out she was married to a Cylon.
10. Michael Hogan as Saul Tigh
An old friend of Commander Adama, Tigh earned contempt when he turned increasingly to drink after the Cylon attacks. The arrival of his flirtatious wife on Galactica complicated things further, and when Tigh was forced to assume command after Adama was shot his command was so slipshod and heavy-handed it incited active resistance from people like Lee. In command of the resistance of New Caprica, Tigh turned to suicide bombings and personally killed his wife, who had traded information to Cavil in return for better treatment of Tigh while he was incarcerated. Afterwards he led vigilante tribunals that executed collaborators. Ironically his discovery that he was a Cylon made him a more committed and loyal officer than ever.