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Rei Ayanami  is the First Child, and the pilot of Unit 00.  At the start of the series, she is shown to be socially withdrawn, seemingly emotionless, and remote, with her only apparent relationship being with Gendo Ikari. As the series progresses, she and Shinji grow closer It is eventually revealed that Rei is a vessel for the soul of the Angel Lilith, and was created specifically by Gendo to be used as a tool for accomplishing Instrumentality Soulless clones of her are kept hidden in the deepest levels of Nerv headquarters to be used as the supposed "cores" of the Dummy Plugs and as replacement bodies for Rei if she should die.

Source from Wikipeida