Rei is ... basically Gendo's first victim. Cloned over and over again as part of Gendo's plan, reduced to an empty husk. There's this scene in Rebuild were she says she's vegetarian - i think it's not meant literally, but that she simply doesn't eat at all except for pills (see the fact that she only takes the tiniest sip from the miso soup Shinji offers her - could ofc be because the broth isn't veggie, but i think it's meant to imply that she doesn't require food and isn't used to consuming it). All she does is sit in her dark, empty apartment, take her medication and do what she's supposed to do. She's not even depressed like Shinji, she is completeley depersonalized. She is nobody. Just a cog in the machine that literally gets replaced when it gets broken.

And then she gets to learn what it's like to be human, to connect to people, to lead a good, non-alienating life when she's working in the rice fields to provide for the community. And she takes that opportunity without hesitation, in the sweetest, kindest way possible. In spite of being a complete stranger to life outside of NERV, she's never scared or confused by all these things she doesn't know. Completely unlike Shinji and Asuka, who constantly push people away either by retreating or by aggressing. It's as if Rei's got this talent for caring and nurturing that the other pilots lack completely. That she's more capable of building a meaningful connection to others in spite of having zero learned social skills.

Which, now that i think about it, may be kinda problematic, as she's a clone of Shinji's mother and in spite of never having been a mother herself, that automatically makes her the archetypal mother (see her deep curiosity about the gravid cat that lives under the train waggon). As if even a clone of his mother has no defining characteristics besides being maternal.

Fundamentally, Rei is the critique of the docile obedient waifu. When we first see her, she is literally, physically unable to stand on her own. She didn't make decisions, she followed orders. She didn't live for herself, she lived (and repeatedly died) for Gendo, believing dogmatically in his vision over her very existence. She's expressionless and unnervingly doll-like because she's deeply traumatized, barely a person. This is the barrier preventing her from making genuine connections with others, which is the uniting theme of every character's individual conflict.

The culmination of each character's story in 3.0+1.0 directly addressed their core conflict. Asuka found a way to love herself instead of seeking external validation (imo this suffered because she spent so much time moving the plot that she didn't get room to breathe), Rei developed the ability to make her own decisions and value her own experiences for her sake, and Shinji stood and faced reality (and his father) instead of running away.