I kind of feel he's the weakest of the main four. He has some strong qualities (his relationship with his sister, his Mysterion persona) but those are rarely shown, and in general, he can't carry an episode like Stan, Kyle and Cartman, in no small part due to his muffled speaking. I've heard claims that Butters has replaced Kenny in recent seasons, but even in earlier seasons, Wendy was effectively the real "fourth member" of the group.
He's not a bad character by any means, and he has quite a few standout episodes, but most of time, he just doesn't have a lot to do.
