Rioton, yeah. She's a couple of years younger than the rest, can't dance very well, can't sing very well, and, according to the backstage parts of their first live DVD, is really nervous and lacks confidence. So she really stands out from the other members.
Having said all that, she is cute, and I think is well liked and I think brings a lot of character to the group. Without her they would overall be stronger performers, but something would be lacking. Her "uselessness" is kindof a charm point in a way?