The original Battlestar Galactica was the quintessential 70's science fiction series: shiny metal robots with glowing red eyes would climb into spaceships and attack decent human beings who were just minding their own business. The aliens in question, which were called Cylons, spoke through vocoders: the monotonic catchphrase "By your command" was constantly imitated by an entire generation of young television addicts. The Cylons were thoroughly evil and inhuman, and the viewer was expected to cheer whenever one of their ships crashed in flames.
When the series was re-imagined in 2005, the most dramatic of many dramatic changes was that the robots were now almost completely indistinguishable from human beings. "They look like us now," the commercials hissed menacingly. Gone was the black-and-white morality of the original show; the viewer of the new series swiftly learned that Cylons were technically inhuman, but might not actually be... inhuman.
The re-imagined series began with a "miniseries" that essentially served as a four-hour pilot episode. The first Cylon that the viewer meets is a stunning blonde woman in a slinky red dress and stiletto heels, played by Canadian fashion model Tricia Helfer. In the very first scene, the woman appears at a diplomatic human-Cylon meeting that the Cylons had been boycotting for many years -- a metaphor, perhaps, for the years that the viewer has gone without new episodes of the series itself. She kisses the confused human representative just as his ship explodes. This is Number Six, the quintessential "new" Cylon.
One of the very first scenes in the miniseries makes it abundantly clear that we are no longer in the 1970's: the spine of the naked Cylon, riding her human lover, glows red as she climaxes. Six is portrayed as cunning and merciless, using a human scientist as a sex toy, manipulating his access to government computers so that she can carry out the massive nuclear attack that all but destroys human society. In another early scene, she kills a human infant just to see what its soft flesh feels like, assuring the viewer (falsely, as it turns out) that anthropomorphic robots are just as single-dimensionally evil as their shiny chrome brethren.
This Cylon, unlike the others who are eventually revealed in the series, is not given a real "human" name for quite some time. We aren't even told what name she goes by when she and Baltar are dating, before the Cylon attack; presumably he called her something other than a number (though, given his general misogyny, perhaps not). We are told, however, that she is the sixth model of twelve, and she is subsequently known as Number Six.
The Cylons have figured out how to create robots that appear human, but their limitation is that they can only make twelve kinds: this means that numerous physically identical Sixes, and Twos, and Fives, and so on, are wandering around the universe. Eventually each model develops a personality. Rather, I should say each model develops a complex of personalities -- in both the literal and the Freudian senses of the word -- because each individual Cylon is shaped by its circumstances. Thus the ones who spend a lot of time among humans sometimes develop an affection for them that their mates don't share, and the ones who suffer a great deal are affected surprisingly deeply by it. Cylons, even Cylons of the same model, sometimes disagree with one another, complicating the assertion in the opening credits that they have "a" plan.
Throughout the series, Number Six keeps some of the characteristics that were demonstrated in the miniseries: she is by far the most sexually charged member of the group, and ironically also the most religious. (In a clever twist on the Mormon themes present in the original series, the human beings are polytheists while the robots are monotheists.) However, since the viewer encounters her most often through visions that Baltar has of her after the attack, it can be hard to tell what she is "really" like. Even the "flesh"-and-"blood" iterations of her model (some of which go by ordinary human names) can be hard to pin down, since they have many of the same qualities we saw in the first Six we met, but shift and change in a way that the "original" Six, frozen in Baltar's mind, cannot.
In this sense, despite the fact that Six is the most visible of the Cylons in the series, she is also the most elusive. Thus, she serves as a good metaphor for the difference between the original series and the new one. The producers of the new BSG make no secret of the fact that their program is a commentary on a complicated post-9/11 world, a world in which the "bad guys" can no longer be counted on to dress up in neatly-pressed uniforms and face us in lines on a battlefield. Today's wars are fought dirty. You can't trust your friends, yet you have to trust your friends or you'll go mad. Put simply: "They look like us now."
In many ways, Six is the most human of the Cylons, experiencing the highest ecstasies (and, we eventually learn, the ugliest tragedies) that we associate with human relationships. And yet, she is the soulless robot that snapped a baby's neck before we even reached the first commercial break in episode one. Some viewers take this as a sign of inconsistent writing, or worse, a misogyny on the part of the writers that matches the in-character misogyny of Gaius Baltar. That might be true, but I also think it illuminates something about how hard it is to write a story about bad guys in the 21st century. The more "like us" the enemies become, the harder it is to paint them as uncomplicatedly evil.