Cavil as Bill Adama's Sister
Trust me, I know this one is far-fetched, but it's amusing enough to write about. First of all, it involves the widely circulated spoilers about the pilot and background for the new series, Caprica, coming out on DVD in April. So don't read this if you are staying away from those.
"Caprica" Spoiler Alert
We're told that the opening of Caprica reveals that the Cylons are created after Graystone's daughter, a computer wizard who has been creating a virtual version of herself, goes with her monotheistic radical boyfriend on a trip, but he suicide bombs the train, killing them, and also killing William Adama's mother and sister. Joseph Adama meets Graystone and they work together to bring virtual copies of their daughters back to life. Long story short, the first three Cylons have the artificial memories of Zoe Graystone, the boyfriend, and Tamara Adama -- Bill's sister. Bill is just a 10 year old boy though.
We've seen nothing of this in BSG. We don't know if it is common knowledge that Joseph Adama helped create the Cylons (not as a programmer but as a mob lawyer who steals important AI software from a fellow Tauron.) It seems if it were common knowledge, this would have made it difficult for Joseph and Bill to prosper after the first war.
Zoe, it should be pointed out, is a monotheist, led to that by a secret school of monotheists. Tamara, we presume, is a standard polytheist/atheist colonial.
Yet we've seen no signs of any Cylon retaining the memories of the early days of their race. There's no reason for that. They should not lose memory, and in fact, they can copy memories from Cylon to Cylon. And as we've discussed, the question of whether the first 8 humanoid Cylons were completely new beings, or if they incorporated the memories of older Cylons has never been settled or even really addressed. The former makes little sense as a deal. The latter should have had more impact. Some of the originals persisted as the Guardians of the first Hybrid, but most were killed by Kendra Shaw's nuke, or left for dead in the area.
So flash forward to the creation of Cavil. It makes sense that some of these oldest Cylon memories would end up in him. Now Zoe, the monotheist (or her terrorist boyfriend, also a monotheist) might be logical choices. But Cavil doesn't seem so religious. So what if he has Tamara's memories? What if Cavil remembers being Bill's big sister?
Well, we do notice that Cavil, who reprogrammed and planted the final five with the colonials, has a big interest in Bill Adama. Shortly after Saul is planted he become's Bill's best friend, and Ellen is planted with Saul, possibly already married. Tyrol is planted to land there too, on Bill (and Saul's) battlestar. Since Cavil's real purpose is to torment his creators, and not kill them, it seems likely he wanted and always planned for the Galactica to survive the war, or so it must be retconned.
This could be because all the Cylons owe a debt to Joseph Adama, not just for their creation, but -- and this is more speculation, though there are hints in revealed spoilers -- because he was also their defender and didn't want them to be enslaved.
But at least one Centurion might have a special connection to Joseph and Bill, with the memories of Tamara. This Centurion would be less likely to be monotheist, even opposed to the monotheists who caused the death of her "original."
Bill, by the way, may not be aware of any of this. He may not know his father was involved, or that his sister's memories were used. He might be quite shocked and incredulous if Cavil tells him:
Cavil: Jospeh never told you what happened to your sister.
Bill: He told me enough! He told me you zealot cylons killed her.
Cavil: No Billy. I am your sister.
Like I said, this is a very low probability theory, but it would make an amusing shock/surprise. It is also surprising how much information leaked out about Caprica (from a casting sheet.) It is unknown if they wanted that much to leak out or not, but it's been confirmed by a variety of sources.
Update: The semi-canonical "Final Five" comic series states that Cavil's memories came from a combination of fragmentary memories from a failed upload of the original John Cavil (Ellen's father) and memories of centurions. As such, memories of Zoe seem almost sure to be in there, and Tamara is also possible, though there was no hint at this in the comic or show.