Aqua wasn't so bad, and Xion was pretty good in 358/2. Namine, much like Kairi had some great potential but basically got sidelined after her initial appearance as well.

I feel part of Kairi's issue was that Nomura basically dropped the whole fairy tale aspect of the Princess of Hearts after the first game, only giving it a bit of lip service in BbS and from what I've heard, KHIII and that basically killed what he probably felt was the only interesting aspect of her character.