I normally consider "fun" as "easy to use". i.e. Mitsurugi, Pyrrha, Nightmare, Patroklos, Ezio, Xiba. In that order. But it's your choice. Try everybody, you might find a gem.
I think Raphael is pretty interesting... I'll get along to him one of these days...
yeah viola was tough to learn but pulling off 25 hit combos are what makes her fun. I'm thinking about Ivy she seems pretty cool and yeah zwei seems like a fun character to be really good with but i'm not sure i dig his playstyle.
Well you like viola, so I guess you like fast characters, if so zwei won't work for you, I'd highly recommend Raphael, Omega, Yoshimitsu or Natsu in that case.