And two and a half years later, I realize that I never did answer the question. This won't be a spoiler -- bits and pieces of this have and will come out in subsequent Steps and nanoSteps, but this is probably the first place outside of my notes it'll be laid out plainly.
Simply put, the skein of Evangelion universes are a zone where both sides in the Big Game played by Heaven and Hell decided to try Something Different, to see if it worked any better than the way they've always done things. They took the roles of two different groups of Precursors (one side with White Moons, the other with Black), and seeded a collection of adjacent "dead" universes, intending to "keep score" at the planetary level rather than the individual level. (Was it a success? In a word, no. End of Evangelion tells the story of one of the better outcomes. Subsequently, both sides agreed to quarantine the entire skein of universes as a failed experiment.)
The Angels are essentially Celestial biological machines -- purpose-made servitor creatures manufactured by the Moons, with divine souls and (mostly) very alien minds. Rei, partaking of their biology, was accidentally (as far as NERV is concerned) created with a divine soul, albeit a very minor one -- and this is the only reason the dummy plug/"backup" system works at all: as she is at the start of Evangelion she has just barely enough Forces to be able to support more than one Vessel, Kyriotate-style. Even so, she's stretched so thin that there's barely enough of her in her "primary" Vessel to be sentient. (An ordinary mortal's soul subjected to what NERV's done to her would simply fall apart from the stress.)
-- Bob
---------
Then the horns kicked in...
...and my shoes began to squeak.
Simply put, the skein of Evangelion universes are a zone where both sides in the Big Game played by Heaven and Hell decided to try Something Different, to see if it worked any better than the way they've always done things. They took the roles of two different groups of Precursors (one side with White Moons, the other with Black), and seeded a collection of adjacent "dead" universes, intending to "keep score" at the planetary level rather than the individual level. (Was it a success? In a word, no. End of Evangelion tells the story of one of the better outcomes. Subsequently, both sides agreed to quarantine the entire skein of universes as a failed experiment.)
The Angels are essentially Celestial biological machines -- purpose-made servitor creatures manufactured by the Moons, with divine souls and (mostly) very alien minds. Rei, partaking of their biology, was accidentally (as far as NERV is concerned) created with a divine soul, albeit a very minor one -- and this is the only reason the dummy plug/"backup" system works at all: as she is at the start of Evangelion she has just barely enough Forces to be able to support more than one Vessel, Kyriotate-style. Even so, she's stretched so thin that there's barely enough of her in her "primary" Vessel to be sentient. (An ordinary mortal's soul subjected to what NERV's done to her would simply fall apart from the stress.)
-- Bob
---------
Then the horns kicked in...
...and my shoes began to squeak.