Developer: NERV Headquarters
Pilot: Rei Ayanami
Evangelion Unit-00, also commonly referred to as Eva-00, is the first successful Evangelion construct resulting from fourteen years of research and development conducted by Gehirn.
Being a prototype model used for data collection and experimentation, Evangelion Unit-00 initially had very limited combat potential. In its original form, colored orange, it lacked compatibility with the various types of armor and weaponry the other Evangelion Units could equip.
It is not until the battle with Ramiel, the Fifth Angel (during which Evangelion Unit-00 suffers heavy damage), that it is rebuilt and improved with combat in mind. This is also when its armor is repainted blue instead of orange. Following the refit, Evangelion Unit-00 successfully fights alongside the other units, and is repeatedly sent into combat against the Angels.
Evangelion Unit-00 is piloted by Rei Ayanami, the First Child. It had taken seven months for her to synchronize with the construct, and it was not without incident. At one point, several weeks before the arrival of Shinji Ikari at NERV Headquarters, Evangelion Unit-00 went out of control during an activation test, nearly killing Rei.
There is a debate over whether Evangelion Unit-00 possesses a Soul like Evangelion Unit-01 and Evangelion Unit-02. Some believe Naoko Akagi's Soul resides within the Core. This is largely because of two incidents which a berserk Evangelion Unit-00 attempts to attack Gendo Ikari and Rei Ayanami.
Supporters of this theory claim that only Naoko Akagi would have reason to hate them both. However, the other Evangelion Units have human Souls because of contact experiments. Naoko Akagi died of suicide, and was not part of such experiments.
Others speculate that Evangelion Unit-00 does not have a Soul at all, or that the Soul might be of a random NERV employee which did participate in a contact experiment at some point during its development. This theory is also feasible since Rei Ayanami has the Soul of Lilith, and as such is universally compatible and can be used to complement any human Soul (albeit in an artificial manner). In this case, it would not matter whose Soul Evangelion Unit-00 contained, if one at all.
However, while Rei's universal compatibility might allow her to synchronize with even Naoko Akagi (despite being murdered by her as Rei-I), there is no reason why Shinji Ikari would be able to synchronize with Evangelion Unit-00 as well. This would only make sense if Evangelion Unit-00 did not have a Soul, and was a blank slate only requiring similar genetic makeup of its pilots. Rei is believed to be a clone of Lilith and Yui Ikari, Shinji's mother, and as such, they have very similar DNA profiles. This theory would also explain why Asuka Langley Sohryu is incompatible with Evangelion Unit-00 as mentioned by Misato Katsuragi (see Episode 14).
Evangelion Unit-00 is ultimately destroyed in the battle against Armisael, the Sixteenth Angel, during which Rei Ayanami (Rei-II) activates its self-destruct, thereby killing herself and the Angel but saving Shinji Ikari and Evangelion Unit-01.