Rei 1

"Rei 1" is a fan-created term (sometimes written as "Rei I") for the first Rei Ayanami clone, who appears for the first and only time in Episode 21 as a child, in flashbacks to the year 2010. She is brought to Gehirn HQ under Tokyo-3 by Gendo Ikari where she is introduced as the "daughter of a friend" who he is taking care of. Naoko Akagi notes the physical similarity between Rei and Yui Ikari.

The night before the base would become officially operational, Rei 1 became lost and encountered Naoko on the bridge of the Command Center. Rei called Naoko an "old hag", and told her that Gendo himself called her that in private; that Gendo's relationship with Naoko was a fraud and he was really just using her for her technical skill. In a fit of frustration and rage, Naoko strangled Rei 1 to death, and then committed suicide by throwing herself onto the Magi supercomputers below.

Notes

• It is very probably that part of the soul of "Rei Ayanami" (actually, the soul of the Angel Lilith) from Rei 1 was used as the soul inside of Eva Unit 00. This may explain the difference in personality between Rei 1 and Rei 2, as Rei 2 was "missing" parts of her soul/personality. For further analysis, see: "The Soul in Evangelion Unit-00".