In the Neon Genesis Evangelion franchise, Evangelions (also referred to as Evas or EVA pronounced "Avangelions"/"Avas"/"AVA"), are the fictional mecha piloted by the Children chosen by the Marduk Institute (a front company of Seele). The Evas were created by Nerv. Their principal purpose is defending Tokyo-3 from the Angel. The Evas were designed by Ikuto Yamashita and series director Hideaki Anno.
Although they appear to be giant humanoid robots, the Evas are actually cyborg, integrating a mostly biological being with a substantial mechanical infrastructure and flexible polymer sheathing and armor plating. But like the Angels, their real defense is the Absolute Terror Field (AT Field for short). AT Fields are almost invulnerable to conventional weaponry, but can be neutralized by deploying an opposing AT Field, allowing the Evas to fight the Angels with greater effectiveness than the conventional army. All the Evas but one are actually derived from the first Angel, Adam (in the Book of Genesis, Eve is created from one of Adam's ribs). Unit 01 is unique in having been born from the second Angel, Lilith. With these revelations, the true purpose of their armor is also revealed - it is actually to constrain and control their movements.
Their monstrous appearance is no accident:
"There's a monster in Japan called the Oni; it has two horns sticking out of its head and the overall image of the Eva is based on that. I also wanted to give the impression that beneath this 'robot monster' image is not so much a robot, but a giant human."
Read more about Unit 01 (Shinji's) at Wikipedia ...
Official Site: Gainax
Links: Wiki-Evangelion (mecha) Absolute Anime Gainax