I have some base. Not much, and not enough to go on, but I have base.
You see, Kaworu is a clone with an unspecified donor. Kaworu is to Seele what Rei is to Nerv. If you ask me, it's too odd that they leave no hint as to Kaworu is. So chances are, just like when you found out that Rei was Shinji's mom, we were in for more mindfuck. It never came though (for unknown reasons). According to logic, there were 3 characters in EVA that played important roles that would be likely candidates. Gendo (because Yui was his wife, no other real reason), Shinji (no real reason here, other than he's the protagonist), and Kaji (who Kaworu looks somewhat similar to). Kaji was also a double agent for Seele, so Seele would have immediate access to him. They killed him after he knew too much, but they also killed him because they didn't need him anymore. Kaji is the best candidate to clone.
Of course, I could be wrong and Kaworu was just cloned from some unknown donor.