Exactly! I'm sure so many other people have said this but that's the problem with trios and just "designated groups" for characters to "belong to", especially for series like Kingdom Hearts with such a wide cast of characters, because those "designated groups" limit character interactions vastly.
Another thing I wanna mention is how even though the BBS trio and Sea Salt trio seem like, well, trios; the destiny trio (or Sora, Riku, Kairi) aren't really a full fledged trio, at least not for the last few games. It's always been a duo and a third wheel, whether that's Sora/Riku with Kairi third wheeling or Kairi/Sora with Riku third wheeling. But I think that just comes with the problem of Riku and Kairi not being a full fledged duo.
They have small interactions here and there, which are fun to watch when we get them, but they're definitely not as impactful as Sora/Kairi or Sora/Riku moments. Which sucks, because they banter off of each other so nicely when we do see them together. But that's just a Kairi problem in general; not having interactions outside of with Sora and when she does, they're almost always *about* Sora.