But maybe not the one you're thinking of. Is there one you're thinking of? I genuinely have no idea at this point.
Quite frankly, it's just this: There are too many party members. I just finished Corel and unlocked Cait Sith, thought 'oh, this is neat. But I'm never going to use him, just like the original game.'
The party at the very start of Rebirth is already pretty well rounded, as far as I'm concerned. Cloud is your excellent all-arounder, Barrett is your ranged tank/support, Aerith is your caster and zoner, Tifa is your fast melee striker, and Red is uh... also a fast melee striker but with more of a focus on defense and parries I guess. Yuffie gets added later and her kit adds a lot of flexible utility due to her ability to attack from range and with any element.
Just in terms of party size, with only 3 members in an active group and only 3 premade slots, there's no space for Cait Sith as far as I can tell. Just for diversity sake I run one party that's Cloud/Cait/Aerith (team 'I struggle with your mechanics), one that's Cloud/Barret/Tifa (team avalanche), and one that's Cloud/Yuffie/Red (the kids table).
This is going to get even WORSE in Revelations, as despite us being down a party member (yeah spoilers sorry) we're gonna get TWO more, and neither of them is gonna replace that missing party members niche (double spoilers) unless maybe they redesign Cait's to make him more magic oriented (three strikes).
Now obviously there's very little Square could do here, as they're adapting an older IP and if they didn't include everyone from the OG game there would probably literally be terrorism in response, and unfortunately Cait's gimmick in the OG was 'silly man with the gambling mechanics'.
Am I just stupid? Do I lack the elite ball knowledge necessary, here? How should I be kitting out/building this guy? I know he has the 2nd highest str after Tifa and the 2nd highest magick attack after Aerith but also like the 3rd worst MP pool, apparently. Right now in addition to the usual purples (an MP up, Steadfast + precision block) I have steal (to exploit his +100 luck), morph (for the gimmick I guess), fortification and empowerment on him.