You're the first person I've seen suggest an idea that has been in my head since the first airing of Maelstrom. And I assume you are making reference to Number Two/Leoben, who in one incarnation or another is, I'm sure, Kara Thrace's (at least biological) father. (Just because Inner Six said Hera is the first human/Cylon hybrid doesn't mean it's true. I mean, she doesn't know who the other five are so anything could have happened and she wouldn't know.)

I don't know if Number Two is also the fifth, or the also 13th, or neither, but I think he has a much more important role to play than we have been led to believe.

The secret of the Cylon yet to be revealed lies in D'Anna Three's reaction in the temple -- one of surprise, but not necessarily fear as she might feel towards Tigh for having tortured him -- and it would seem to me that she has always imagined herself superior to the Cylon yet to be revealed. She then says to Baltar, "You were right," implying that they have discussed the Cylon yet to be revealed ... and implying that we have heard that conversation. While many might see this as implicating Baltar, it really encompasses everyone (human and Cylon, if there really is a difference) that D'Anna and Baltar both have knowledge of. (That is also a point for Felix Gaeta, who has been at the center of everything -- the virus, the Cylon detector, the star charts, the trial -- and he was involved with biogenetics before and plots the course/FTL jumps for the fleet.)

Still, I'm convinced Number Two/Leoben is in there somewhere. He was introduced in the mini-series, he is the most metaphysical and philosophical of the S7, he is bound to Kara, and he exists in a form different from Leoben, as we learned in Maelstrom.