His sideways glance and his smirk are always disturbing. He's good, nothing wrong there! But he is, how could we phrase it - a little weird. Always looks like he's not just a simple Transfiguration teacher.
At first glance he doesn't look like he likes students very much. He reprimands them severely when they make a mistake during the lessons, even the slightest, and his mocking undertones are worse than insults, especially since one can't answer him - or at your own risk.
He's a teacher! But, no jokes, he's a good teacher, although lacking a little patience.