From Battlestar Wiki, the free, open content Battlestar Galactica encyclopedia and episode guide
"Number Six" is a humanoid Cylon who plays several key roles in Cylon society. She is the first humanoid Cylon that viewers witness in the re-imagined Battlestar Galactica. Stunningly beautiful and highly religious, this highly driven model is marked by a ruthlessness against all who stand in her way. Number Sixes are typically sensual and use seduction in infiltration.
To date, more unique versions of the Six model have been witnessed than any other humanoid Cylon. Besides the use of numerous aliases, Six is also the only Cylon model known to have varied appearances and personalities: while most often seen as a platinum blonde, Six has also been seen with honey blonde and black hair in varying styles, and displaying personalities ranging from overtly sensual to wide-eyed and innocent, and from blindly supporting the attack on the Colonies to desiring peaceful coexistence. This differs from the other known models who generally display the same personality traits (Sharon Valerii and Sharon Agathon being exceptions), use the same name, and have fewer cosmetic differences in appearance.
The copy of Number Six that seduced Dr. Gaius Baltar into unwittingly betraying the Twelve Colonies by giving her access to the Colonial defense mainframe. Much later, she came to live on Galactica and fell in love with Saul Tigh and attempted to bear a child by him. After suffering a miscarriage, and as Tigh reunited with his wife, Ellen, Caprica pulled away from him. After this she participated in the Battle of The Colony and settled New Earth with Baltar.
An entity (also known as Baltar's Internal Six or, more commonly, as Head Six) appearing to Gaius Baltar in the form of Caprica Six shows herself immediately after his rescue from Caprica, and later gives him advice (from building a Cylon detector to asserting political power various times), predictions about the future (primarily about the birth of Hera Agathon) and insights into the past events of the human race. This entity, while imperceptible to everyone other than to Baltar and Caprica Six herself, is not affiliated with either the Cylons or Colonials and has repeatedly stated that she is an angel of God. Even after Baltar's death, this entity maintains the appearance of Six one hundred fifty thousand years later when discussing the possibility of the humans repeating the same mistakes as their progenitors.
A copy of Number Six who appears aboard Galactica to try to discredit Baltar as a traitor. The evidence she used was flawed, and he ended up being more trusted than before. After being exposed, she is blown out an airlock by Cavil and presumably downloads into a new body (The Plan).
A copy of Number Six poses as a systems analyst assigned to the battlestar Pegasus, and enters into a romantic relationship with Admiral Helena Cain. After she was identified as a Cylon, she was captured and repeatedly tortured and sexually abused by the crew before escaping with the help of Gaius Baltar (TRS: "Resurrection Ship, Part II"). For a time, Inviere is a leading figure in the Demand Peace movement (TRS: "Epiphanies"). She eventually commits suicide by detonating a nuclear warhead aboard Cloud 9 and fails to download, making her death permanent (TRS: "Lay Down Your Burdens, Part II").
A Number Six copy who is leading a faction that advocates for the search for the Final Five Cylons, introduced in "Six of One". She is eventually killed by Sharon Agathon, distressed with visions of the Opera House and afraid Natalie will take her daughter Hera, with no chance to download (Sine Qua Non).
A Six with curly honey blonde hair and a noticeably more innocent disposition than her sisters, Lida attempts to comfort and seduce Baltar aboard the rebel Basestar after he and President Roslin flee Galactica during the Zarek-Gaeta coup/mutiny. Lida approaches Gaius, initially misinterpreting his feelings of guilt (among the first he's ever shown) for leaving his flock behind as an injury; later, they sleep together. After speaking with her in bed, he decides that he needs to return to his followers (Blood on the Scales).
Sonja is one of the platinum blonde Sixes, similar in appearance to Caprica/Messenger Six. She is elected to represent the rebel Cylons in the Fleet's new Quorum. Her first duty is to request the Cylons be given custody of Sharon Valerii in order to put her on trial for treason (TRS: "Someone to Watch Over Me").
Given her new posistion, the Colonials have accepted at least part of the deal the Cylons offered through Galen Tyrol (TRS: "A Disquiet Follows My Soul") and now have the representation they wanted in the Colonial Government (TRS: "Someone to Watch Over Me").
She is later seen at a meeting of the new Quorum of Ships' Captains, representing the rebel Basestar, as they discuss the impending transfer of Admiral William Adama's command from the aging Galactica to the Basestar (TRS: "Islanded in a Stream of Stars").
Armistice Station Copy
This Number Six is a portent of doom for the peoples of the Twelve Colonies; she arrives on Armistice Station shortly before it is destroyed in what are the opening shots of the Cylon attack in the Miniseries. Dressed in a long-sleeved red tunic and matching knee-length skirt, she is the first and last "diplomat" ever sent to the station by the Cylons since its construction. She greets the Colonial officer present with the question "Are you alive?" When he answers that he is alive, she tells him to prove it - by kissing her. After he realizes that Armistice is under attack, she smiles and forces another kiss on him shortly before they are both destroyed.
Later, three Sixes in the same outfit are among the group of Cylons that rescue and debrief Aaron Doral, following his stranding on Ragnar Anchorage by the suspicious Colonials. One of them responds to a comment of the Eight in the group with "By your command," a trademark Cylon phrase in the Original Series.
It's possible that Caprica-Six may be one of the Sixes who rescues Doral, as she would have had time to be downloaded; however, the chronology of events in "Downloaded" make this extremely unlikely. It is far more likely that the Armistice Station copy might have been one of them.
At least three additional variants of Six have been operational on Caprica, and appear to lead Centurion squads and have a subservient role to the "overseer" Six variants. This variant of Six is distinguished by the white raincoat she wears.
One of these variants distracts Helo, enabling him to be captured. In this, she shares the same seductive characteristic as shown by her sisters at the armistice station and with Baltar - her first act on "freeing" the captured Agathon is to ask "Are you alive?" and kiss him. She is subsequently shot by the Caprica copy of Sharon Valerii (33).
The second acts as an observer to Helo's "escape", and this same variant may the one leading the Centurion troops into the Caprican farm Helo and Valerii are hiding in (The Hand of God).
Another Number Six in this role boards Pegasus from a Heavy Raider along with a group of Centurions during Pegasus's attack on a Cylon communications relay. She is shot and killed by Kendra Shaw, who then exposes the Pegasus network administrator Gina Inviere as another copy of this Cylon model (Razor).
When the Centurions suddenly stop their attack on the Caprica rescue team and the Caprica Resistance, the Caprica copy of Cavil visits one of these variants of Sixes who was presumably commanding the Centurions and she informs him of the decision to have a truce with the humans. When Cavil offers to deliver the message himself, this Six informs him that his model voted against the truce, but he still decides to deliver the message as he is sympathetic with the humans. (The Plan)
On Caprica, Six performs the role of a Cylon overseer, working with a copy of Number Five (Aaron Doral) to ensure their experiment involving the stranded Karl Agathon and the Caprica copy of Sharon Valerii either reaches its desired conclusion, or is suitably terminated (Bastille Day, Act of Contrition, Litmus, Flesh and Bone, Tigh Me Up, Tigh Me Down). These copies of Number Six wear a militaristic black outfit, with a black body-suit underneath suited to martial arts.
Overseer Six demonstrates a certain vindictiveness towards Sharon, giving her a beating that goes far beyond the needs of the experiment and dismissing her belief in Agathon's love (Litmus, Flesh and Bone). The Five working with Six suggests that she is jealous of Sharon's chosen role in the proceedings, and Six does state that Agathon is "very handsome" (Bastille Day, Litmus). Six maintains that her dislike stems from Sharon acting like a human, which essentially makes her a human in the scheme of things (Flesh and Bone).
An overseer Six is present at the Cylon's established base at Delphi, and is briefly seen by Karl Agathon when he attempts to gain access to the base in order to steal a ship and get off the planet (Colonial Day).
Six surprises Kara Thrace in the Delphi Museum when she returns to Caprica to retrieve the Arrow of Apollo. They engage in an all-out brawl, reminiscent of the earlier beating of Sharon. Just as Six appears to be winning, Thrace rushes her, knocking both of them off of a ledge. The Six falls underneath Thrace, breaking her fall, and is impaled on building debris, killing her (Kobol's Last Gleaming, Part II).
An overseer Six is present at the "farm" where Thrace is taken after she is found by the Cylons after being left behind by the resistance during an ambush. While the Cylon doctor Simon operates on her, Six is there to relay orders and see that everything was proceeding smoothly. She notes that Simon seems to regret Thrace's impending move to processing, and he replies that this is none of her concern. This copy is later bashed over the head with a fire extinguisher by Thrace as she makes her escape (The Farm). (Note: a deleted scene from this episode shows a second Six in the same black outfit at the farm, taking instructions from the other copy. Thus, it's hard to be certain that the Six speaking to Simon was the same one brained by Thrace.)
One of these copies views D'Anna Biers' broadcast in a theater on Cylon-occupied Caprica (Final Cut). She remarks that the resilience displayed by the Colonials is impressive, and seems overjoyed that Sharon and the hybrid baby survived.
A Number Six in a gold tunic is one of four 'rebirth nurses' who greets the reincarnated Caprica-Six (this takes place shortly after the Cylon attack has run its course), and it is likely this same group who nurse Boomer as she returns to life several months later. The Six in the group welcomes Boomer home and refers to her as "little sister".
Several other copies of Six are seen on Caprica in a variety of different outfits.
This Six is not given a name onscreen, but is referred to in the script as "Tough Six". She typically wears a revealing chains-and-leather outfit and has brown hair with blonde streaks. This Six lives in the fleet and works as a prostitute. She is part of a Cylon meeting held by Brother Cavil, also including Leoben Conoy, Aaron Doral, and Shelly Godfrey. She later has a sexual relationship with Cavil, and gives him a sarcastic rundown of their fellows' failures. When Shelly Godfrey is exposed, she takes her place long enough for Shelly to escape (and be airlocked by Cavil) before escaping herself by presumably simply removing her disguise once far enough from the marines as she has a different hair color and style. Her fate is unknown as she is not killed on-screen, but she may have rejoined the Cylons after the settlement of New Caprica and likely became one of the rebels (The Plan).
New Caprica Copies
Various copies of Number Six form part of the Cylon occupation force that arrives on New Caprica one year after the Colonials settle in there. Throughout their four months stay there, they fall victim to various different attacks from the resistance movement.
When the resistance begins using suicide bombers, the Cylons decide to employ stronger measures against the human population. A vote is taken by six of the seven Cylon models on Colonial One, and while Caprica-Six disagrees with this, two other copies overrule her. This implies that Caprica-Six no longer has the same amount of power she used to have, not even with those of her own model (Precipice).
Another Six is drowned by Jean Barolay after taping her mouth and throwing her into a septic tank at New Caprica's water treatment plant, where she works during the occupation. Despite attempts at therapy after her resurrection, this Six bears lasting emotional trauma as a result. Roughly a year later, she is on the rebel baseship and meets up with her murderer. A brief altercation at the Raptor occurs, where she kills Barolay who shows no remorse for her actions, instead glibly offering to kill the Six again. Natalie kills this copy immediately after Barolay's murder, thus appeasing the Colonials they have allied themselves with. Just before her execution, the Six—knowing that her actions cannot be without consequences—admits that she is glad that Natalie, whom she views as a sister, will be the one to kill her permanently, as with no Resurrection Ship she is unable to download into a new body (Faith).
The Sixes vote along with the Number Twos and the majority of Number Eights to stop the lobotomization of the Raiders (Six of One). Several Sixes are members of the Cylon rebels. This includes Natalie and the Six who kills Jean Barolay. The latter one is distraught about her actions, and is subsequently killed by Natalie with Anders's sidearm (Faith).
Moreover, a number of them are Heavy Raider pilots alongside a number of Eights and these Sixes fly in battle alongside the Colonial Viper pilots in the battle to destroy the Resurrection Hub. They tow the Vipers into battle, allowing them to launch a surprise attack and fight alongside them against the enemy Raiders (The Hub).
Since Gaeta's Mutiny at least, some of the Cylons have joined in CAP and are even briefed by Starbuck in the briefing room. They are shown flying missions with Colonial pilots as an allied patrol, and flying solo missions in search of a habitable homeworld simultaniously with, and identical to, Colonial Raptor pilots. An example of the former is a Six copy who participated in the interception of Boomer's raptor, and identified Boomer's voice as that of an Eight. Some are also seen applying the organic resin to Galactica and walking freely about her passageways, and even participating in the human ritual of posting photographs of lost colleagues in the Memorial hallway.
Though serving in the integrated, allied CAPs under the command of Captain Kara "Starbuck" Thrace (CAG) and the Colonial military chain of command, the Sixes (like the pilot Eights other than Lt. Sharon Agathon) continue to wear black Cylon flight suits without insignia and do not appear to have been commissioned as Colonial officers. Neither do their pilot collegues appear to have bestowed individual call-signs upon them as yet. Nevertheless, the Sixes, Eights and Colonial pilots are intermixed in the briefing, and Starbuck distributes the flight assignments through a Six. They are addressed by their model number, making "Six" their de facto surname (Someone to Watch Over Me).
Yet more variations of Number Six's personality manifest themselves among the Cylons assigned to try and repair Galactica. A dark-blonde Six with a working-class/"knuckle-dragger" demeanor (or, at least, a learned demeanor) gets into a heated argument with one of Galactica's maintenance crew and has to be physically restrained by a Number Eight; soon after, however, this Six sacrifices her life to save the human from a breach (Islanded in a Stream of Stars).
Infected Basestar Copies
During a mission to investigate a basestar infected with a deadly disease, Gaius Baltar sees a number of Cylons, including Sixes, dead or dying from an infectious agent. One of these is a Number Six with black hair, who explains that the basestar brought onboard a mysterious beacon and the Cylons became sick afterwards. Baltar strangles her to death in a panic after she blames him for what's happening. This Six and the other ones who had died on the basestar's deaths are permanent as when the other Cylons came to investigate, they purposefully left the Resurrection Ship behind so the dying Cylons wouldn't download the virus into the rest of the Cylon population (Torn). Later, another surviving Six is found in the infected baseship by Galactica, and becomes a part of their plan to infect the Cylon race with the virus spawned from the ancient beacon. However, she and the other Cylons are killed by Karl Agathon before they are within download range of a Resurrection Ship, thereby killing this Six permanently (A Measure of Salvation).