Purpus, you're right on. Rika's strengths are that she has a lot of emotion behind her voice and more often then not she CAN hit the right now, but there's just not enough control and power to keep the note steady and consistent. We can cut her some slack because of her asthma which makes it hard to take much air into her lungs and diaphram which is then expelled to produce a powerful sound, but lots of vocal training will help improve with that.
Especially now that she's fronting Biyuuden fulltime, she's going to need to get some serious training because so far it seems that Erika and Yui have started to surpass their leader on vocal skill.