OMG imagine them climbing a mountaim and Viola is having trouble geting on top of a ledge and Zwei offers his hand and she takes it and then it cuts ti someone else
He's been the main character for SC until now so I'd say he's too iconic to not be in. Since he's the leader of the Schwartzwind and he once wielded SC, I'd say he has story reasons to be in. Gameplaywise, I believe he self taught himself and his style is unique so there's another good point slayer =)
As for Zwei... I'm worried. He got stabbed and he did fall down a pit :'(