Sir Toby Belch is Olivia’s uncle and something of a minor-league Falstaff. A penniless drunkard who sets stock by his nobility of birth, with a taste for pickled herrings that likely leave him flatulent, he makes himself quite at home at his niece’s.
He has the full confidence of Sir Andrew, whom he considers a fool, and can play him like a charm. This allows him easy access to Sir Andrew’s money. He has managed to convince Sir Andrew that he has a chance at Olivia’s hand. Though most of what Sir Toby does appears to be purely for his own entertainment, he is quite capable of moments of genuine nastiness, reminding Malvolio of the difference in their social class, tying the steward up and having him locked in a dark room, putting Sir Andrew in situations that could lead to his serious injury or death, and in the end telling Sir Andrew to his face what he is. Sir Toby is quick to pull his sword, though we have no evidence as to whether he has any prowess with it. Taken with Maria’s wit and prank-planning ability, he marries her.