I agree completely, OP! There's nothing wrong with finding a character boring/believing her to be lacking development/etc. I honestly think Kairi needed more character development myself (and Sora/Kairi is my KH1 OTP). But it's not right to actively hate her for "being a bitch," "acting like a slut," "destroying all good relationships in the game," etcetc. Not only are those terms terribly offensive, their intended meanings are completely wrong. Kairi never treats her friends unfairly, never tries to seduce anybody or cheat on Sora (who she is 1% away from dating), and while Sora/Riku (or any other pairing that isn't Kairi/Sora) isn't canon, if she had even the slightest inkling that they liked each other, I'm sure she'd give them her blessing.

Everyone has their own taste in characters, and you can dislike who you want, but character bashing? Never cool. Especially when the character has done nothing you accuse her of. :/