Rei Ayanami is a half-angel hybrid clone created by Gendo Ikari using the DNA of Lillith and Yui Ikari. She is intended to trigger the Third Impact or to prevent a Third Impact from occurring outside of Gendo's scenario. Terminal Dogma, the deepest part of NERV, contains cloning tanks which hold backups that allow her soul to be recovered and her body to be replaced in the case of her death.
Differences in Personality between Canon and Warhammer
In Canon, she slowly gains fondness for Pilot Shinji as the series continues, but remains loyal to Gendo until the last moments of the series. In the improvement, she becomes a devoted follower of Shinji Ikari after he names Unit 01 "Principio Eternus" and gains a greater will of her own when he lends her the Warboss figurine -- which represents the Id, basic drives and impulses that she did not show inside Canon. She becomes more conscious of her own life and importance and is less willing to give it up, though no less willing to dive into danger for Shinji's sake.
Rei Ayanami shows a posessiveness of Pilot Ikari. She enjoys provoking Asuka and creates emotional chaos in her wake while remaining seemingly unaffected. She is willingly part of the Harem which forms around Shinji and, despite her rivalry with Asuka and covert possessiveness, seizes opportunities to expand it for what she believes to be Shinji's sake. It is implied, once and completely out of nowhere, that she has kissed Shinji, possibly more than once.
Rei continues holding the illusion of being under the sway of and following the orders of Gendo Ikari, to her own demise on one occasion. Gendo remains uncertain as to how much Shinji has changed Rei and her willingness to follow his orders.
Rei Ayanami, being part angel, can generate her own AT Field while outside of an angel. At the urging of the Warboss, she uses this at a low level to harass local thugs to the level that Yakuza bosses are terrified of her. Shinji Ikari and Maya Ibuki can tap into this effect to gain her powers without triggering the Magi's AT Field sensors. While this ability allows her some control over her sync ratio with Unit 0, it also inhibits her ability to use her Eva's AT Field. Unit 01 is also the weakest of the Evas and for a significant time she is unable to do anything other than provide fire support to Unit 01, -Principio Eternus-. Once Unit 0 is remodeled to allow her to participate in combat fully, she dominates Shinji and Asuka for quite some time despite remaining a fan of heavy weapons.