Yeah Anno
basically gave up on Rei development-wise after episode 6.
I see it more that Rei's role in the narrative is one where the element that she plays is essentially existing as a foil of who Gendo becomes after he's forced to give up Yui. He's constantly locked himself into doing what he has to do as he sees it, so Rei acts as an extension of that as the clone that he wants to connect with but forces himself to be detached from.
While that does make her interesting and mysterious, that's also why that moment Anno mentions where she learns to smile is really all that there is to get her into place insofar as understanding. She's a blank slate, and she sort of has everything that she needs to set about learning to do things in her own way – which is VERY much unlike all of the history of psychological damage that Shinji & Asuka are exploring. Later, she's also essentially the opposite foil in Shinji's mind, instead of seeing his mother & father, he sees Rei & Kaowru because that's the concept of safety and familial bonds that he's learned to relate to. For better or worse, Rei is closer to being a part of Evangelion's setting than one of its characters simply because the nature of her existence is a foil by which the characters understand themselves, rather than something like Ghost in the Shell that is more about exploring the nature of her conceptual existence.
Given that role on top of the fact that Asuka was initially planned as the main character, but then they changed
Evangelion to have a male lead, so that it wasn't too similar to Anno's previous work
Nadia,
especially since it was originally imagined as a sequel to that series. It's not really hard to see why he really focuses his attentions on the two of them very differently than how he does on the rest of the characters in the series – because of how they're reflections of one another, and the foils by which they're both forced to face their own inner trauma, which is really at the heart of what Anno is pouring into
Evangelion.
That's the good and bad thing about the series is that it VERY much isn't a narrative that's designed for people who are looking for a series that dives into all of the characters and what's going on with them, to tell a story – but one that focuses very particularly on the elements that it wants in order to just convey a particular type of emotion through the experience, which is... also a lot of what
Gunbuster is like.
X