Zwei is a setup-oriented character whose play style isn't for everyone. I enjoy him as a secondary character, and I do well with him. I can't stay up close with Zwei for long, he's more about spacing.
And while he isn't in a lot of tournament play currently, give it time. This game is very young. I contend upper-tier play is dominated by series established characters because the players have a shorter investment time in them.
On a character design note, he would have been far more interesting if, instead of Zwei being a wolf, his summons was a mirror/ ghost/ spectre of himself.