Evangelion Unit-00 was the first Eva ever created. It was the proto-type. 22 days before Shinji came to join NERV, an activation experiment had gone wrong and Eva Unit-00 went berserk. Eva Unit-00 ejected the entry plug where the pilot, Rei Ayanami was and the plug fell to the ground. Eva Unit-00 was not created to battle angels, but it's enhanced upgrade is.
Dr. Ritsuko Akagi believes it was mental instability in Rei that caused Eva Unit-00 to go berserk. In a battle with the 5th Angel, Ramiel, Eva Unit-00 was severely injured when Rei tried to protect Shinji who was piloting Eva Unit-01 and Eva Unit-00's armor and paint had melted. NERV repaired Eva Unit-00 with a blue paint job, up-graded abilities, and shoulder blades.