Mr. Twit
As his name suggests, Mr. Twit is a twit. He is also an incredibly evil, stone-hearted, and strange man who is profoundly ugly - on the inside and out. In fact, Mr. Twit's hair covers virtually all of his body, hair which is mostly spiky and unkempt.
Mr. Twit also eats old food of his hair when he's hungry instead of having a more traditional snack. Beyond that, he is unbelievably sadistic and treats the Muggle-Wumps horribly. Mr Twit is, in other words, a violent and heartless slob with a penchant for overindulging on alcohol.
Mrs. Twit
Like her husband Mr. Twit, Mrs. Twit is a horrible individual who lives up to her rather unflattering name. And like her husband, she is incredibly horrible on the inside and out. After years of being so incredibly negative, her beauty deteriorated and she became an ugly woman.
Unlike her husband, though, Mrs. Twit is petty and vengeful. She plays game with objects around the house to mess around with her husband. But she engages in more violent and cruel and vengeful acts with the Muggle-Wumps and other innocent people/things. She is, in other words, a horribly mean and evil person with no redeeming qualities.