This article contains some discussion of suicide.
Of all the ways the 2004 reboot of Battlestar Galactica reimagined the classic sci-fi show, none stood out as strongly as the humanoid Cylons. The Battlestar Galactica of the early 2000s took what had been a heavily mechanized sentient race of robots in the 1978 series and turned them into humanoid Cylons that were indistinguishable from human beings. Not only did this lead to some of the show's best moments when fan-favorite characters were revealed to be Cylons, but the lore behind these covert androids was central to the Battlestar Galactica reboot.
The tension of searching for the Cylons among the human survivors helped to make the 2000s run of Battlestar Galactica a success, and understanding the thirteen Cylon models was key in appreciating the show's many complex nuances. The humanoid Cylons come in two groups: the original "Final Five" who came into existence two thousand years before the show begins, and the other 8.
Number One: John Cavil
Most Cylons in Battlestar Galactica repeatedly took the same name, and became known by humans as that name. For Number One, this name was John Cavil, played by Dead Stockwell. Cavil was the first Cylon model created by the Final Five in the Battlestar Galactica timeline, and he assisted in the creation of the other 7.
Of all the humanoid Cylons in Battlestar Galactica, John Cavil was by far the most vengeful and sadistic in his hatred of humans and his makers. Although he didn't subscribe to the monotheistic religion of the other Cylons, he twice posed as a clergyman, possibly in mockery. During the Cylon Civil War, the Fours and Fives, as well as one of the Eights, sided with John Cavil.
John Cavil was revealed in Battlestar Galactica season 2, episode 20, "Lay Down Your Burdens." Number One, as Brother Cavil, was on Galactica and had been counseling a deeply disturbed Chief Galen Tyrol. When Starbuck returned, Tyrol noticed a copy of Brother Cavil among the rescued Caprican survivors. He alerted security, and Brother Cavil admitted to being a Cylon. Later, the Resurrection Hub was destroyed, and John Cavil planned to kill Ellen Tigh to reacquire the secret to resurrection. He failed to do so and took his own life rather than fall into enemy hands.
Number Two: Leoben Conoy
Number Twos became known as Leoben (Callum Keith Rennie), and they were one of the more religious Cylon models in Battlestar Galactica, believing in mysticism and spirituality rather than literal interpretations of Scripture. One Leoben had a particular obsession with Starbuck (Kara Thrace), because he believed she had a special destiny.
The Number Two humanoid Cylons in Battlestar Galactica, sided with humanity, believing that their ultimate spiritual destiny was to find a way for the two species to live together in peace. The Leobens were revealed as Cylons on the Ragnar Anchorage space station in the 2003 Battlestar Galactica miniseries (which served as a pilot for the 2004 show).
A Leoben, claiming to be an arms dealer, was discovered by Commander William Adama, who noted he was already sick from the radiation surrounding Ragnar, something the humans were immune to, but the Cylons were not. After a struggle, Commander Adama of the Galactica killed Leoben. At the end of the 2004 Battlestar Galactica series, a copy of Leoben was seen on New Earth, having agreed to remain with the humans.
Number Three: D'Anna Biers
D'Anna Biers, the Number Threes (played by Lucy Lawless), first appeared in Battlestar Galactica, season 2, episode 8, "Final Cut." D'Anna was on the Battlestar Galactica posing as a reporter for Fleet News Service, covering the Gideon Massacre and life onboard the ship. D'Anna discovered that Sharon Valerii was alive and pregnant. Commander Adama ordered D'Anna to remove the video evidence as it could jeopardize the fleet's security. The episode ended with a group of Cylons watching the documentary and the deleted footage, thus revealing D'Anna Biers as a Cylon.
The Number Three Cylons in Battlestar Galactica were calculating and duplicitous, often manipulating humans and other Cylon models if they felt it warranted. The D'Anna's obsession with the Final Five in Battlestar Galactica led to John Cavil boxing them. When he did eventually unbox D'Anna, she killed him. Once the Resurrection Hub was destroyed, this version was the last remaining copy of the Number Threes.
Midway through season 4 of Battlestar Galactica, after having found Earth to be an irradiated wasteland, Commander Adama invited the Cylon allies to join them in a search for a habitable planet. D'Anna stayed behind, however, preferring to die on Earth with her ancestors.
Number Four: Simon O'Neill
The Number Fours, often going by Simon O'Neill (Rick Worthy), were the medical specialists of the Cylons in Battlestar Galactica. The Number Fours were the most machine-like of the iconic Cylons, with a personality that was calm to the point of apathy. His use of logic and reason paired with his lack of emotional response made him appealing to Number One/John Cavil, who viewed the Simons as a tool.
The Number Fours were revealed in Battlestar Galactica, season 2, episode 5, "The Farm." Kara Thrace had been shot on Caprica and woke in a hospital. She was tended to by a doctor named Simon, whom she suspected of being a Cylon, a concern he dismissed. At one point, Simon called her "Starbuck." She was certain he'd had no way of knowing that was her handle and faked sedation to sneak out of her room. She saw Simon talking to a Number Six and killed him when he returned.
Number Five: Aaron Doral
Revealed in the 2003 Battlestar Galactica miniseries, The Fives (Matthew Bennett), or “Dorals,” sided with the Ones and Fours during Battlestar Galactica's Cylon divide. They were emotionally unstable and manipulative, switching from amiable and friendly to angry and violent. The Dorals had an "every-man" appearance, which made it easy for them to blend in with the fleet.
The reveal that Aaron Doral is a Cylon came when Gaius Baltar, under pressure from Commander Adama and President Roslin to create a Cylon Detector, pointed to Aaron and called him out. Ironically, this came at the suggestion of another Cylon, Caprica-Six. Later, after Aaron had been thrown into the brig, he vehemently denied he was a Cylon, though due to not being a sleeper agent he was aware of his true identity.
Number Six: Caprica-Six
The Number Sixes (Tricia Helfer), along with the Number Eights, were the humanoid Cylon models seen most in Battlestar Galactica, and they were the first revealed. In the Battlestar Galactica miniseries, Caprica-Six had seduced Gaius Baltar into allowing her access to the defense mainframe, which led to the destruction of the Twelve Colonies. Shortly after, a model of Number Six known as Messenger Six, or Head Six, began to appear to Baltar.
Caprica Six, after helping to free Athena and Hera from the Cylon Basestar, was kept in the brig on Galactica. Colonel Saul Tigh began to have a sexual relationship with her, which ultimately resulted in the first Cylon-Cylon pregnancy. When Ellen Tigh returned, jealousy led her to fight with Caprica Six, causing a miscarriage. All remaining copies of the Sixes, including Caprica, joined the Battlestar Galactica humans on New Earth.
Interestingly, Number Sixes were the only humanoid Cylons in Battlestar Galactica to show notable differences in appearance, with different hair colors and styles, as well as notable differences in personality. The Sixes were also not known by one particular name, but rather by different names for different copies. Each typically used seduction as a means of infiltration, and all the Number Sixes were deeply religious. They tended to feel sympathetic toward humans and were part of the rebel faction in the Cylon Civil War.
Number Seven: Daniel
The Number Sevens, referred to briefly in Battlestar Galactica as "Daniel," were sabotaged by John Cavil. They had been artistic and sensitive, close with Ellen Tigh. This led to John Cavil becoming murderously jealous. He corrupted the amniotic fluid the Daniels were housed in, thereby eliminating them altogether. The Number Sevens never appeared in Battlestar Galactica. It was believed they were named after Dr. Daniel Graystone, who created the first Cylon, as depicted in the Caprica prequel series.
Number Eight: Boomer/Athena
The Number Eights (Grace Park), most often known as Sharon Valerii, were expert infiltrators, though they were also known to be more empathetic to humans, and one copy, Athena, was even able to gain the trust of the humans who knew she was a Cylon. However, another copy, known as Boomer, was a sleeper agent posing as a Colonial fleet Raptor pilot. After the Cylon attacks, she began to perform unconscious acts of sabotage.
Suspecting she may be a Cylon in Battlestar Galactica, season 1, episode 8, "Flesh and Bone," Boomer insisted that Baltar try his Cylon detector on her. While the test was positive, Baltar withheld the information. Shortly after, she attempted to assassinate Commander Adama in a shocking twist. From there, her journey eventually leads to her death in the war against the Cylon rebels.
In contrast, Athena was aware that she was a Cylon. Athena was first encountered in Battlestar Galactica by Karl "Helo" Agathon. He had been stranded on Cylon-occupied New Caprica, and Athena pretended to be Boomer coming back to rescue him. While her intentions at first were strictly Cylon-oriented, she fell in love with Helo and tried to help him escape. Back on Galactica, it was discovered that Athena was pregnant with Helo's child, proving humans and Cylons could breed.
Final Five: Samuel Anders
Sam Anders (Michael Trucco) was an athlete who'd been training on Caprica when the Cylons attacked the Twelve Colonies. He was rescued by Kara Thrace, whom he later married. On New Caprica, Sam was a leader in the Resistance, along with Galen Tyrol and Saul Tigh. After the Colonials abandoned the colony, Sam, believing his wife to be dead, decided to join the fleet as a pilot.
Sam's identity as one of the Final Five Cylons was revealed in Battlestar Galactica, season 3, episode 20, "Crossroads." He, along with Tory Foster, Tyrol, and Tigh, had been hearing hallucinatory music and were drawn into the Galactica gym where they realized they were Cylons.
Sam decided to try and retain as much of his human life as possible. However, after a bullet wound to the head, Sam became a hybrid, an entity that served as the central computers of Basestars. He was submerged in liquid and attached with wires to the Galactica, which allowed him to control other Basestars in the Colony when Galactica invaded. In the final episode of Battlestar Galactica, Adama has Sam pilot the fleet into the sun so that those who remained on New Earth could begin again.
Final Five: Tory Fosters
Tory Foster (played by Star Trek Discovery star Rekha Sharma) was first introduced in Battlestar Galactica, season 2, episode 17, "The Captain's Hand" where she became President Roslin's aide. Toward the end of Battlestar Galactica season 3, she began to deteriorate, and like Tyrol, Tigh, and Anders, she came to the realization that she was a Cylon. Tory embraced this far more than the others did. She felt liberated from her old life and sought out new experiences, such as drinking and having an affair with Gaius Baltar.
In Battlestar Galactica, season 4, episode 3, "The Ties That Bind," Tory murdered Galen Tyrol's wife Cally because Cally had learned the four of them were Cylons. In "Daybreak, Part 3," Colonel Tigh volunteered to share resurrection technology with John Cavil, and Tory became nervous that in the sharing process, the rest of the Battlestar Galactica's Final Five would know what she'd done to Cally. When Tyrol saw her memory of murdering Cally, he strangled Tory.