there are many secrets within evangelion, the most tuanting and confusing is Rei herself, but others include the evas, the angels, and nerv. read on already!
Rei isnt human, i guess if u saw the series u know this. however, rei isnt an android either, which makes this confusing to many viewers. rei has many clones, but each clone is the exact same, which is a problem. the real rei was killed, then a "dead" clone was made. the clone survived, then grew up as the real rei. then the dummy plug experiment started, which came with the need to clone more rei. all rei after the original carry no soul, which is why having exact clones is a problem. this is why rei always caused eva to go berzerk. eva needs a soul to sync, and rei couldnt give it one. this is also why the dummy plug program went as far as it did, it was needed in order to utilize the first child, rei II. the final rei is rei III which is the used model for the dummy plug system, especially the one in eva unit 1.
the evas are copies of the first angel Adam. they are also the closest humanity has ever gotten to god, but yet they are used to destroy. they need a soul to sync, and 14 yr old children obviously have the best souls, or else they would use anyone, this is confusing but never explained. they can use an absolute terror field, which is a protective barrier around everyone, giving them an identity (see existance). however the evas AT fields are much more potent and exaggerated. this is all known about the evas, except that eva unit one contains the soul of Yui Ikari, shinji's mother.
there are 18 angels, adam, lilith, the 15 seen on the series, and humanity. each has its own individual characteristics which make it deadly, but also beautiful, loving, etc. each angel is also one of the original possibilities of what humanity coulve been, another oddity. they only have one goal, to unite with adam. however, they all fail, but one, humanity, can come close thanx to the technology known as Evnagelion.