in the Neon Genesis EvangelionEach Eve contains a soul and is usually the soul of the pilot's mother. Shinji's mother Yui and Asuka's mother Kyoko are both present in the Eva and have been demonstrated throughout the anime. In the case of Rei, whose soul is Eva-00?
First of all, Rei is not a naturally born person, but a clone made with the DNA of Yui Ikari and the angel Lilith. So he doesn't have a mother and Yui lives in Shinji's 01 unit. So what soul is hidden in unit 00? Probably Rei's soul itself.
In episode 14, Shinji and Rei take the cross compatibility test to sync their Eva. Usually a pilot is only compatible with his own Eva, but if Rei 1's soul was really in the Eva-00, Nerve probably thought that Shinji could also be compatible with the Eva-00, since Rei is a clone of her mother. Rei actually manages to synchronize with the Eva-01, but Shinji cannot control the unit 00, probably because the soul in the unit 00 consists of Yui, but also partly of Lilith.
When Shinji tries to control Eva-00, it gets angry. The target of the fists of unity is the glass behind which Gendo and Ritsuko are standing. Misato believes that the target of this attack is Rei, but Ritsuko himself believes that she is the real target of the angry Eve.
The soul of the first Rei, who has her reasons to want to attack Ritsuko, who is present at both tests, certainly lives in Eva-00. Ritsuko is blonde but looks a lot like her mother Naoko, who had black hair. In episode 14, the scenes from Unit-00 see Ritsuko with dark hair. It almost seems as if Rei 1's soul mistook Ritsuko for Naoko. the one who killed Rei Ayanami.
This spectacular Rei cosplay cannot be overlooked by the way! If you're a fan of Evangelion, this giant Prime Studio Eva 01 figure is waiting for you!