First Children: Ayanami Rei

Rei, pilot of EVA-00 is an enigmatic figure. Her origin is unknown, she speaks very little and seems to never show any emotion, though she seemed to have a special connection to Gendo, and later to Shinji as well.
Later in the show it is revealed, that Rei isn't a real human being, but a clone of Ikari Yui and Lilith, born to fulfill Gendo's own version of the Human Instrumentality Project.
She showed tremendous mental strength and fearlessness in the face of death - which is no longer surprising as there are several Rei-clone. If one is killed, another one takes her place.
Rei I was killed by Akagi Naoko when she still was a little girl, Rei II was killed when she she initiated self-destruction of Unit-00 to kill Armisael. Rei III finally became aware of who she was, developed into a "real" person - she then merged with Adam and Lilith to initiate the "forbidden marriage" and start Third Impact.
The dummy-plug system essentially consisted of Rei-clones.