I think the idea of overrated and underrated is a bit too vague for me, even after reading all these posts.
But if I had a character I think Nomura is overusing in situations better suited for other characters, I think it's Sora and Riku.
Of course, I like both characters, but Nomura has this terrible habit of pushing them into scenarios where I feel other characters are better suited?
The best example is the end of MoM. It's a perfect example of just how overused they are. Kairi, an underused character gets outdone yet again by these two for no reason other than... I mean, I don't even know the reason. It still makes no sense to me.
I believe Namine is also a great example of an underused character. I admit, I feel like when it comes to Namine, Roxas, Xion, and others like them, Nomura wrote himself into a corner. The "no one ever dies" corner. It's a great plot thread to think "Hey, they were their own people with their own hearts!", but now what do you do with a character like Namine, you know? Even in situations that seemed to fit her perfectly (Namine looking for connections between Kairi's memories and Sora to find him??), she wasn't used sooooo
There are other characters floating around in my head right now, but I guess I'll just say Nomura is very uneven with the attention he gives to each trio, and if you aren't in a trio, you're screwed. I think pretty much all the KH characters have something interesting, cool, and fun about them that rarely gets utilized these days.