Armisael

Armisael is an "angel of the womb". He appears in episode 23 as a giant ring of light, but soon proves to be able to change it's form at will.
He manages to get into direct contact with Unit-00, infecting it and trying to take control of it in doing so.
As Asuka is not able to synchronize with her EVA any more, Rei under attack from Armisael, there is no other choice than to send out Unit-01, which was locked away after the incident with Zeruel. But as that also proves to be useless, and now Armisael tried to merge with Unit-01 as well, Rei initialised Unit-00's self destruct sequence whilst remaining in the Eva maintaining its AT field, holding the angel in place.
The Angel is destroyed but with high costs, Unit-00 is lost and Rei is killed ... again.