Soul

Every living being in Neon Genesis Evangelion has a soul. The soul is everything that makes up a living thing's being. Souls in NGE originate from the Chamber of Guf, and enter & leave through the Doors of Guf. The light of the soul - the "ego" barrier - is known as the A.T. Field. A soul needs a vessel in order to create life. Lilim are made of souls which have combined with LCL where the soul shapes the LCL into an appropriate vessel. When a person's A.T. Field collapses, the soul is freed and the vessel returns to its original state of LCL. Adam, Lilith, and the Angels are made of a substance known as as Particle Wave Matter or PWM which can hold its shape even without a soul. However, souls are able to shape PWM as seen in The End of Evangelion when Rei returns to Lilith who then takes on Rei's form.