In the recent GameInformer interview, Nomura said they included Final Fantasy characters in Kingdom Hearts because it was a new IP and they thought Sora, Riku and Kairi needed help standing out on their own. He's said this before in another interview, but I can't remember which one it was! Does anyone remember?

I don't know if this is the one you're looking for, but in an interview with USGamer prior to KHIII's release, this was said

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Final Fantasy characters have always been a big part of Kingdom Hearts, but it was always fun to watch them interact with Disney characters in interesting ways and of course it brought together Disney fans and Square fans. But it appears Kingdom Hearts 3 won't be using Final Fantasy. I'm curious to know why you went in that direction? Is it because you wanted to focus on the core cast, because it's their final game?

Nomura: I can't disclose too much about the Final Fantasy characters or whether or not they'll be in Kingdom Hearts 3, because I can only speak to what's been shown. But the reason you haven't been shown any Final Fantasy characters is that there are so many original characters that need to resolve their own problems and issues. And because their issues and problems are very difficult, there just hasn't been that much space to fit Final Fantasy characters so far." When you look back on the first Kingdom Hearts, Sora was still a new character, so we kind of had the Final Fantasy characters as supporting characters. But now that it's been 15 years, the original characters are more polished than they were before, so I don't think think they need the Final Fantasy characters help as much as they did before. [...] Also, when we first released Kingdom Hearts, there weren't that many other titles that brought together the Final Fantasy characters. I actually think that Kingdom Hearts may have been the first to do that. But nowadays, there are an abundance of titles that do that, so I don't see the value of having that kind of feature in the game anymore. But that doesn't mean that I'm saying that there won't be any Final Fantasy characters. Please look forward to future information.

Here's the full interview: https://www.kh13.com/news/usgamer-interviews-tetsuya-nomura-on-kingdom-hearts-iii-development-pirates-of-the-caribbean-gummi-ship-system-and-much-more/

And in the KHIII Ultimania, there was an interview with Nomura where he said something similar again.

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Q: No characters from the Final Fantasy series appear in KH3, can you tell us the reason for that?
A: Put simply, they just couldn't be accommodated. There are too many characters that appear in the main story, so as a result the appearance of FF characters wasn't able to be prepared. Technically, polygon models were created for Leon (and some others). Apart from Leon, some other characters, like the Fairy Godmother from Cinderella, were made, but in the end they just weren't able to be used, which made some of the production staff angry.

Full interview here: https://www.kh13.com/news/spoilers-the-big-tetsuya-nomura-interview-from-kingdom-hearts-iii-ultimania-has-been-translated-in-full-r2953/

I hope this helps.

It was the USGamer interview! I don't know why it was so hard for me to find. Thanks so much for your help!