Humanoid Cylon speculation
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The Cylon evolution into twelve distinct humanoid models introduced a serious problem for the remnants of humanity in the Fleet. Early into the search for Earth, it seemed that anyone could be a Cylon, raising doubt and suspicion within the human populace ("Tigh Me Up, Tigh Me Down").
Fans of the show are rife with speculation about who could be a Cylon infiltrator in the Fleet. Viewers were introduced to the "Significant Seven" over the course of the miniseries and the first two season, but the nature of each of these characters was revealed shortly after that character's introduction.
In season three the speculation by both characters and fans turned to the Final Five, enigmatic, almost-mystical figures who seem to have a link to ancient Colonial history. The nature of the five, according to Moore, is "fundamentally different" from the other seven models.
At the conclusion of "Crossroads, Part II" four recurring characters are revealed to be Cylons. The revelation that long standing characters such as Colonel Tigh and Chief Tyrol are Cylons blows the field wide open; if they could be "toasters," then anyone could be the last Cylon. In particular, Tigh has a long and well-documented character history dating back to the first Cylon War.
Clues from Official Sources
During a Q&A session on the official Sci Fi channel Battlestar Galactica forum Ronald D. Moore notes that he already knows who the last Cylon is, and that he's already left clues as to who it is.
Bradley Thompson was asked when the writers knew the identity of the final five:
General factors for consideration
In terms of narrative, to prevent an anti-climax it seems likely that the final Cylon would probably need to be a more prominent character than those of the Four. This centers the spotlight in the central "human" characters (Roslin, William and Lee Adama, Baltar and Starbuck). However, there is still half a season for characters to develop and gain significance with viewers.
With the Final Five being "fundamentally different" from the Seven, extrapolating any characteristics we learned of Cylons from the Seven (including physiology, loyalty or self-awareness) is of questionable validity.
Colonel Tigh has a well documented history stretching back at least to the first Cylon War. Also considering the Temple of Five it seems Final Five Cylons somehow significantly pre-date the Seven.
It has been seen that Cylons can start families, however it is unclear how the Five came to exist. Having known natural parents would seem to preclude one being any more than half-Cylon.
Of the 11 so far there have been only four female models. The final Cylon being female would serve to equalize this gender bias.
The first Hybrid gives a monologue about his knowledge of the Final Five in the extended version of "Razor". It states that the fifth Cylon, which is still in shadow, will come toward the light, hungering for redemption which will only come in the howl of terrible suffering.
In "Revelations", D'Anna Biers, mentions that there are only four of them in the Fleet. If this is true, then this would narrow the field to those characters who are on the rebel basestar at the time, characters previously assumed dead or missing, or other more esoteric suggestions. However, it is uncertain if the statement can be taken so literally. The characters who are on Galactica or other ships of the Fleet include Lee Adama and Kara Thrace, while Laura Roslin, William Adama and Gaius Baltar are on the Cylon baseship at that time (Revelations). An alternative interpretation of "in the Fleet" could refer to loyalty and/or self-identity rather than physical presence; Number Three could have been telepathically aware that one (i.e., Tory Foster) had embraced her Cylon identity, thus leaving four others still loyal to humanity.
The colonies' president has been unquestionably a boon to humanity's search for earth, indeed the two times she has been removed from office have been the two greatest setbacks in the journey.
Roslin has also experienced prophetic dreams and visions (those induced by chamalla, and a dream she shared with the Cylons Caprica Six and Sharon Agathon). She fits very well the profile of the scroll's dying leader, the scrolls also having links to other more mystical elements of the series, including the Temple of Five.
However her dying nature, from an aggressive form of breast cancer is also the biggest cause to doubt Roslin being a Cylon. It is unknown whether the Final Five can succumb to cancer, but Cylons seem to be generally tougher than humans. Baltar's cure in "Epiphanies" was probably designed based on the assumption that Roslin's physiology is human.
If William Adama were revealed to be a Cylon, it would be one of the biggest shocks the series could produce. Overall Adama appears to epitomize a very human set of strengths and weaknesses. Nevertheless, he does experience hallucinations in the fifth Razor Flashback, although he sees and hears people in his environment instead of seeing himself in a different one. The experience could also be explained by stress or other factors, however.
The Adama family is well documented. His father, Joseph Adama, was known to Romo Lampkin. The proposed spin-off Caprica, will feature the history of the Adama family, including a young William. This may be difficult to square with any revelation about William Adama being a Cylon.
Could Lee's chronic angst be partly due to guilt at his Cylon nature? Was the Leoben who said "Adama's a Cylon" (Tigh Me Up, Tigh Me Down) referring to Lee? (Note: The writers hadn't decided on the identity of the final Cylon at that point. Moreover, if Adama were a member of the Final Five, Leoben would not know this.) He might project in "Resurrection Ship, Part II", although this is more likely an oxygen deprivation induced hallucination.
The posting of his then-wife Anastasia Dualla as Pegasus's executive officer, despite her lack of seasoning and non-qualification as an aviator, while presumably mitigating the concerns about a junior having undue influence with the commander, also mirrors the placement of Galactica's XOs: Colonel Saul Tigh, a Cylon is romantically involved with a Six, and Captain Karl Agathon is married to an Eight.
Lee Adama is told that he is "a son trying to flee his father's shadow" by Romo Lampkin. This conceivably could link with the description of the final Cylon as "still in shadow, drawn toward the light."
In his defense, Lee is also a member of the well-documented Adama family. A "switched at birth" scenario could explain this, as could the "fundamental differences" between the Final Five and the Significant Seven.
Baltar's vast intelligence, weak character and lack of loyalty to anyone but himself have made him ultimately humanity's (unwitting) betrayer. Baltar himself longs to be a Cylon as this would absolve himself, however comes to the conclusion that he is human after all. Baltar certainly has a lot to be guilty about, whether he has enough of a conscience to be truly "hungering for redemption" is questionable, however his confession to Roslin seems to indicate that this is the case (The Hub).
There are good reasons to suspect Baltar. He exhibits vast intelligence that may be superhuman in nature. His Virtual Six, her precognitive abilities, and the apparently projected environments they often co-experience are suspicious, as well as the fact that both Virtual Six and later the Hybrid tell him that he is "chosen".
Kara Thrace's path towards her destiny has involved many strange, perhaps supernatural events. Whether this destiny involves her being a Cylon is a matter of speculation (indeed members of the Significant Seven, who do not know who the Final Five are, tell her that she has this destiny), but it is certain there is something special about Thrace herself.
Certainly Thrace exhibits phenomenal natural skill as a pilot, and manages to learn to fly a Raider exceptionally quickly (You Can't Go Home Again). From childhood, Thrace has been drawing the mandala from the Temple of Five. However she was also physically abused as a child and still carries the scars; Cylons seem to be more robust than humans and possibly may not carry such long-term injuries. The same may be said of the knee injury which put a stop to Thrace's Pyramid playing ambitions.
Most strikingly, the ace pilot apparently dies in "Maelstrom" but returns (resurrected?) in "Crossroads, Part II". Furthermore, not being on Galactica at the time, Thrace is the only major "human" character who has a reason for not reacting to the music which, to the "four", reverberates around the ship. However the final Cylon may, for some reason, not have heard it, or may have ignored it.
Thrace has engaged in combat with Raiders on many occasions without any incident similar to the one which occurs between Anders and the Raider during the Battle of the Ionian Nebula, however it is possible that the Raider was only able to identify Anders because he had already been "activated".
None of the above?
The January 2008 edition of Entertainment Weekly showcases a two-page photo of the cast for Season Four posed in a manner similar to Leonardo DaVinci's The Last Supper with an explanation of each character's pose/position provided by Ron Moore. Near the far left of the table a space is left vacant and a cup sits on the table marking the absent character's intended place. Moore tells EW that "We have not yet revealed the final [unknown] Cylon." and when asked if there was a possibility that the final Cylon was indeed not seated at the table Moore notes "You ferreted that out pretty slyly. I didn't really want to give that away." William Adama, Laura Roslin, Lee Adama, Gaius Baltar, and Kara Thrace are all present in the picture, as are Final Five Cylons Saul Tigh, Galen Tyrol, and Samuel Anders.
However, the importance of this photo is seriously put to question given Aaron Douglas' comments on the photo. According to the actor, he was unsure that Moore had anything to do with the photo, given that Moore was not present for the shoot. Additionally, the photo shoot had at least six different setups and the actors had changed places during the photo shoot, thus contending that there is no hidden meaning for the photo. Moore, however, later explicitly confirmed that the final Cylon is not in the picture.
Based upon the mutterings of the Hybrid in "Razor", as well as Ron Moore's claim that he has already left clues to the final Cylon, one could assume Tom Zarek is the last Cylon to be found.
Tom Zarek likes to move from second-line positions, is not big on being in charge but always craving to be noticed and understood, is constantly trying to make people understand why he did the things he spent time in jail for, and has the "cleanest" past (no known relatives, etc.). His criminal background and political ambitions fit with someone "hungering for redemption."
Her first name "Anastasia" is the feminine form of "Anastasius", ancient Greek for "resurrection". Her surname "Dualla" may be derived from Latin word Dualis, from which originates the English words "dual" and "duality", such as a Cylon living as a human - or a Sagittaron serving as a soldier (The Woman King).
The rejection of her people's pacifism and her failure to reconcile with her father over their final heated argument (Final Cut) allude to the redemption angle.
At the time of the initial Cylon attack, she immediately threw herself at the special assistant to the president and promptly developed a romantic relationship with him. Despite having never undertaken the years of formal military education of a colonial officer, she managed to maneuver herself to receive a commission and a vital posting as the executive officer of Pegasus, and to become become related by marriage to both the military and (later) civilian leadership of the Fleet. Her posting as Pegasus's XO, despite her lack of seasoning and non-qualification as an aviator  while presumably mitigating the concerns about a junior having undue influence with the commander (her husband), also mirrors the placement of Galactica's XOs: Colonel Saul Tigh is a Final Five Cylon and romantically involved with a Six, and Captain Karl Agathon is married to an Eight.
She is the by far the most prominent "human" remaining who is not of substantially (real-world Earth) European ethnicity. Edward James Olmos wears blue contact lenses to portray Admiral Adama as non-Hispanic, and Hot Dog (portrayed by Olmos's son, Bodie Olmos) and Felix Gaeta (portrayed by Italian Alessandro Juliani) are very blanco Hispanics if not European. All other prominent non-European characters are either dead or Cylons: East Indian Rekha Sharma as Cylon Tory Foster, African Rick Worthy as Cylon Number Four / Simon, Korean Grace Park as Cylon Number Eight / Sharons, African Lorena Gale as the late Elosha, and Eurasian Lily Duong-Walton & Alexandra Thomas as the half-Cylon Hera Agathon. In this day and age, it seems unlikely that the writers would make all of the prominent "ethnics" who are allowed to live be Cylons.
After Anastasia Dualla, Gaeta is the most prominent character not depicted in the Last Supper photograph. His double-agent (or, implied by Baltar and Sweet Eight, triple-agent) cross-collaboration on New Caprica, his perjury at Baltar's trial, and his morphine addiction all leave him seeking redemption.
A counterargument against Gaeta, however, is analogous to that against Dualla. Whereas Dualla is the most prominent member of a racial or ethnic minority still depicted as alive and presumably human, Gaeta is the most prominent sexual minority so depicted. His former lover, Louis Hoshi, is a very minor character, Admiral Helena Cain is dead, and all other known gays/bisexuals are or were Cylons (Gina Inviere, Caprica Six andDownloaded Three).
Sharon "Boomer" Valerii is a unique case, given that she has broken away from her model line by voting for the lobotomization of the Raiders, for the first time in Cylon history. Further, she has grown to disdain humanity as a result of what transpired on New Caprica, and, while given the duty to take care of Hera Agathon, she later attempts to kill Hera as she views the child as a mistake.
Valerii has since made it a point to go against the grain of Cylon society, inciting a civil war and accompanies Cavil to the Resurrection Hub to resurrect Number Three in the attempt to stop it. Further, she satisfies many of the pre-existing conditions, including:
Like the four known members of the Final Five, Valerii was ignorant of her Cylon nature.
The only downside to this theory is that Valerii is part of the Number Eight model line. However, given that the Final Five are "fundamentally different", this yet to be explained fundamental difference may be able to explain this. Further, none of the Final Five have model numbers, and are thus outside the Significant Seven's mainstream; the Five may have infiltrated Cylon society as well in some way, just the same way Tigh and the others were able to infiltrate human society dating back to at least 30 years pre-Fall.
The virtual beings (possibly including the virtual Leoben) may actually be a single entity, the final Cylon being more spiritual than corporal. Virtual Six claims to be an angel from the Cylon God, which could be roughly correlated with the "the five priests devoted to the one whose name cannot be spoken" from Pythia. The overall actions of these beings seem to be neither pro-Cylon or pro-human, which fits with the Final five being a third party in the dynamic of the battle.
None of the other final five have shown similar characteristics to this, and there is no particular reason to suggest the final Cylon is greatly different from his or her four "siblings". When Number Three sees her vision of the final five in the Opera House (a vision repeated for other characters, thus not simply in her mind) they are five humanoid figures in white robes, at least one of which is known to her.
More Esoteric ideas
The final Cylon is