Theory:Kaworu's Origins

From The NGE Fan-Geeks Project

During the mysterious Contact Experiment that occurred shortly before Second Impact, a human "donor" made some kind of physical contact with Adam. The relevant flashback in Episode 21 includes the panicked announcement, "The DNA that dived into Adam has already undergone physical fusion!" The exact meaning of this is undisclosed, but details in Episode 24 -- such as Kaworu's date of birth (the day of Second Impact) and the way he identifies Adam as the "mother" from whom he was "born" -- heavily imply that Kaworu is a product of the experiment. If so, Kaworu is potentially (despite outward appearances) a hybrid entity that wavers somewhere between Adam and Homo sapiens on a genetic level.

Both the show and all official materials remain silent on the identity of the donor.