I dont think any of the characters qualify as a Tsundere. It's a stretch, but Ritsu probably is the closest thing to one, but she certainly isnt. I'd put her on par with Kagami from Lucky star. Who isnt really a tsundere as well.
The only time Ritsu may have qualified was during that terrible anime-only jealousy flare-up. Both Mio and Azusa are closer to the tsundere type otherwise.
