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Basically, just do some rewatching.
Later revelations shouldn't invalidate all we have seen before - they were present all along, we just didn't know about them. Besides us knowing, they don't change anything, they just explain what we already saw.
So instead of going "We learnt that Rei is X, so she's probably/I guess it would make her Y", we should ask ourselves, "Is Rei Y, and if so, could it come from fact X we know about her?"
Don't assume how anything we learn might affect her; Look back and see how she is, in fact, affected or not.
What kind of journes does she go through, and how does it intersect with those of the other characters, if at all?
Not all, but many of these questions can be answered by a trip to wherever you stockpile your DVDs.