well she seemed to be able to handel herself pretty well at the end of KH2 and she cant relly gain any character development being stuck on an island waiting for her friends, her telling them she is no longer going to wait around ( like she did at the end of kh2 ) and go help fight is really the only way for her character to grow, as she travels with Riku ( who if becomes a master can take her as his aprentice witch would be cool) shell gain the experiance needed to help deafeat Xehanort once and forall, as for the "shocking ending" that could be a shock to some, or it could be something like Sora getting stuck in the ROD, or something incredible crazy like Mickey getting posesed by Xehanort lol. but yes Kairi does need to actually do something, she needs to be out in the worlds with them, and not because she was captured.