Leoben is pretty much a crazy religious fanatic. He loves the Cylon religion and god, and in the process throws doubt and misinformation everywhere.
I've pretty much just considered his entire character to be based in irony. He is fanatical about monotheistic dogma, and yet does these horrible things. He believes in truth and faith, and yet spreads lies and doubt (the rape pregnancy not really being true, just a child, etc.) to break people towards his beliefs.
You know? I didn't really think there was anything much to it beyond that. Just a Cylon that has pretty much gone beyond humanity and become a monster. I think he ultimately represents a human who has taken religion too far, but I consider all the cylons to basically be humans who have taken a certain aspect of humanity to the extreme, ultimately failing to reach the mark of surpassing their creators.
edit:
Also I seriously think its either Adama or Baltar. Baltar had that scene with D'Anna or whatever in the temple where he went "You were right", so idk. Plus he's been accused of using Cylon projection. But Adama and Tigh, have been friends forever, so maybe Adama has been one for the longest time and befriended one of the seven. Kind of like Boomer and Tyrol.
The only problem with that is that D'Anna couldn't immediately identify the fifth, but that might just be memory loss. Also Leoden said Adama was a cylon and he lies all the fucking time.
(This
could be Lee Adama tho...)
Plus there's the whole "only 4 cylons in your fleet", and yet that could of just been a lie by D'Anna as well. Especially if she couldn't really make out the 5th.
It could be Starbuck too, but again its MR PSYCHO telling her she'll destroy humanity so I don't believe that very much. Someone else said it too tho, so idk. It's possible because she came back from the dead, but she could be the avatar of Aurora, which could be the destiny. The virtual Leoben inside Starbuck said he'd take her to the place inbetween life and death and when she was revived she said she found Earth too, sooo
Ultimately I think Starbuck is going to lead humanity and Cylonity (lol) to a god, which will probably fuck them up since they are all NO...DIVINITY...DOES NOT EXISTS. Maybe the fifth Cylon is actually a god, possibly the jealous god that destroyed Kobol or overthrew the others or whatever. It would sorta explain how the 13th tribe knew Cylons would exist later on down the road besides prophetic vision.
There's the possibility that the 5th cylon is the child between Tigh and Six as well. Living in the shadow wanting to go to the light, ie being in a womb wanting to come out and seek its redemption. This could fit Lee too since he was said to of wanted to move out of his father's shadow. Or Adama (Adam) being the 1st cylon ever (pretty doubtful). Who knows!!
Whoever it is they probably have ties to the colonial history (Temple of Five), so it might not even be a person we know at all and might just be FIRST CYLON THAT HUMANS MADE NO ONE SPECIAL or maybe it will be.
I really want it to be William Adama though. Badly.
edit edit:
There's also the whole Lords of Kobol and Cylon God connection to think about too.
edit edit edit:
fuck. I just remember the Last Supper photo, and RDM said that the last cylon is none of the people in the Last Supper photo which means Roslin, both Adamas, Baltar, and Starbuck aren't it, unless if he was lying.