That's just the thing, though. You barely EVER get to see Kairi's dynamic with Riku or any other characters, for that matter, and I hate how the SoKai love interest just feels so forced. At least with the BBS trio you didn't have to worry so much about romantic implications.
And as for them being flat, I dunno, man. Ventus has a lot of depth to his character.
First, there's the fact that he hasn't seen the outside world prior to BBS (that he can remember), so he handles things differently. When his friends fight in Radiant Garden, it's a really harsh blow because he doesn't understand -why- friends would fight. It's clear that he looks up to Terra as his friend and brother, and a lot of Terra's personality has rubbed off on him, like his blunt sense of humor. More over, it's clear that Ventus has a fair bit of insight when it comes to sentimentality, which he probably got from his Master and Aqua- maybe a little from Terra, too.
And because he's so sheltered and gets babied all the time, Ventus really hates being treated like a kid. Kid's also a bit rash, etc, etc, I could go on.
The same goes for Terra and Aqua, really. They have their own quirks and character flaws that I feel make them much stronger characters, especially Ventus and Aqua. Sora was fine in KH1, but KH2 left him with absolutely no character development, and he's just become so boring, IMO. Maybe it is unfair, but TAV and even Riku I just find to be more likeable.
