If you have been watching BSG, then this isn't really a spoiler. All the clues have been there, so it is just a matter of connecting the dots. If you have not been keeping up with the show, or you just haven't figured it out and are waiting for show to spell it all out, don't read any further; or at least don't bitch at me if you can't stop yourself from reading.
Starbuck is the first Cylon-Human hybrid. She beat Hera by a couple of decades.
Ever since she was a little girl, Starbuck has been drawing and painting something that looks very much like the pre-supernova that marked the way to earth.
Leoben has long sensed that there is something special about Starbuck, but he hasn't known quite what it is.
Starbuck came back from the supposed grave. Oddly, she died in a storm on a planet they were exploring, and her body wound up on Earth a long way away; and nobody can call that 'normal'.
Starbuck's father taught her, as a little girl, the very song that united the 'final five' and restored their memory of being Cylon.
Starbuck spent almost an entire episode talking to someone who wasn't there. Her father. Who can only be a Cylon:
Yes, Starbuck's father is the 'Thirteenth Cylon', long separated from the 'Twelve Cylons' much the same way the Thirteenth Colony was removed from the Twelve Colonies. (All this has happened before, and will happen again...) Technically, he is referred to as the Seven, but the symmetry of him being the Thirteenth Cylon revealed to us is too meaningful to ignore. In Episode 15 of Season 4, it was revealed that the 'final five' helped the Cylons build eight humanoid Cylons. They referred to the missing Cylon model as "Daniel". Daniel is described as artistic. Music is an Art. The amniotic fluid used to grow the Daniel copies was poisoned, and the DNA template was corrupted; but I think the original Danial escaped and lived as Starbuck's father. At least, that is where he lived until Starbuck's mom tried to make him give up music and he left.
So, where did he go after the split? I have an idea about that too. I think he moved on, and had at least one other child. At least one other Human-Cylon is out there. One that also has conversations with people who are not there, which was explained in Episode 16 as 'Cylon Projection'. Yes, you guessed it: Baltar
Baltar was considered the genius of his generation, far smarter than other humans.
Baltar has mental conversations with a Caprica 6 Cylon who isn't there, just like Starbuck.
Caprica 6, real and projected, feels that Baltar is special; much the same way Leoben feels about Starbuck.
Baltar and Starbuck are Human-Cylon hybrids. It's the only thing that explains their strange experiences and maintains narrative consistency.
As further support of the link between Hera and Starbuck, as fellow half-breeds: Hera just happens to draw on a piece of paper the notes to the song that Starbuck's dad is trying to coax from here memory as they sit at the piano. The very song that makes the Final Five come running over to her demanding to know where she learned it.
Oh, one more thing... That makes Baltar and Starbuck are half-brother/half-sister to each other.
Anyone want to bet a shot of scotch on it?