Hoppy said: Yeah it's hard to mess up something like this but JC needs to hire someone else other than Rie for any tsunderes. Maybe Saki Fujita who voiced Ayano in Yuru Yuri and Tsurara in Baby Princess.
I've read a bit of KMB, and while Rie could easily handle Sonya (she's got more range than she's credited with) it doesn't seem like a role they'd automatically place her in. Not sure I'd call her terribly tsundere... there's a bit of that, but not much. I'd say good odds on someone else getting the role.