I really like the idea that Cait Sith is an autonomous AI, that Reeve has a sort of VR environment where he can assume control and take over communicating through Cait Sith any time he wants or needs to. As a result of that interaction, Cait Sith has a fingerprint of Reeve about him even moreso than normal, that's the result of his experiences and interactions that make him unique. Cait Sith and Reeve get to be the same person – but also are unique expressions of that person that are distinct from one another.
Ghost in the Shell: SAC does a lot of this sort of thing in how it explores the individuality amongst the Tachikoma who share all of the same information, but don't have all the same curiosities. They do a lot of wondering about the nature of their own existence as well in that, and you get attached to them and feel a finality to the damage to their physical shells, because there are specifics about them that could get lost forever if the body gets destroyed even if they can technically come back. I think would fit Cait Sith – especially in the transition between Cait Sith 1 & 2, and that's a really REALLY good area to expand FFVII more in a way that would be engaging and add a lot more depth to a character who very much needs it, not to mention help to make his sacrifice feel different than the other character losses, but also not one that's devoid of meaning or sadness by having a replacement that's able to arrive.
I think that Reeve would be most interesting if we learn that he's got a passion for exploring the idea of machines that can think on their own. It's clear that Shinra is only really interested in things that are loyal, programmable, dependable, and expendable. That would help frame him as someone who's working in the place that he needs to in order to actually do that kind of work, but feels a frustration around something deeper that goes beyond just his personal motivations, and reflects itself in who Cait Sith actually is as their own entity when they encounter the party.
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