Funny, I automatically thought Fuu = Ravenclaw, but let's hear Lin's opinion on the matter. Now you mentioned it I can tell her natural sneakiness (though it fades a lot by the end of the series) would make her qualify for Slytherin quite well.
IDK. Umi's heroic but with a very specific purpose and has a strong mean streak I find most of the Griffindor-sorted characters lack? Again, I can't claim to be an expert on the subject. She's SO NOT RAVENCLAW though. I know that much.