Evangelion Unit-00 (エヴァンゲリオン 零号機[?], "Zerogōki") is the first functional Evangelion unit created, serving as the prototype for the rest of the Evangelion series. It is piloted by Rei Ayanami.

Unit-00 is the Prototype, the first successful Evangelion, and is piloted by the First Child, Rei Ayanami. The head design is similar to that of the previous failed attempts, but with only one camera. There are two domes on the top of the head, one white and one transparent green; their function (if any) is never revealed. Unit-00 is the only one of the original Evangelions that is never seen without its armor (Unit-02 was revealed in The End of Evangelion, and Unit-01 on multiple occasions) or opening its mouth; thus the appearance of its organic body is unknown.

Unit-00 was initially colored orange like the previous prototypes, but its armor was damaged beyond repair by the Angel Ramiel's particle beam. The subsequent repairs and upgrades included blue armor plating in addition to other alterations made in order to make it more suitable for combat conditions. It was later refered as Evangelion Unit-00' (エヴァンゲリオン 零号機(改)[?], "Evangerion Zerogōki Kai").

The upgrades to Unit-00's armor make it look more like the subsequent Evangelions, but there are still several differences or actual limitations. Shoulder pylons are added to the Unit's armor; though similar in appearance to the ones on Units 01 and 02, they are used to hold retro-rockets, as seen in the fight with the ninth Angel, Matarael. Because it is only the prototype, Unit-00 cannot be fitted with D-Type Equipment. The true nature of the soul inside Unit-00 is never specified, but it is implied to be directly connected to Rei in some fashion. Unit-00 was self-destructed by Rei in order to destroy the sixteenth Angel, Armisael.