I'm guessing because he's a white haired pretty boy that Hauptmann Brand is a bad guy...
Well not so much a bad guy, he just has a difficult job to do and is super serious carrying it out and his sister's as of yet undisclosed antics kind of put him into something of a difficult position when it comes to dealing with her.
Or at least so the Implications in the Battle of Britannia manga and his interactions with his Alstede are any indication.
He's not so much a 'bad guy' as his is just a guy trying to balance his job and his taking care of/putting a leash on his sister. Can't always judge a book by it's cover as they say.
