Sir Toby Belch is Olivia’s boisterous and disreputable uncle. With his love of drinking and practical jokes, he embodies the revelrous and chaotic spirit of Twelfth Night. His surname Belch is indicative of the comical and often crude role he occupies within the play; he is as quick to reference his late-night carousing, or to burst drunkenly into song, as he is to engage in the kind of clever wordplay that is beyond the understanding of his more foolish friend Sir Andrew.
In terms of cleverness, he finds his match in Maria, and the two work together to make a mockery of Malvolio, for whom Sir Toby acts as a foil. While Malvolio values rules, rigidity, and order, Sir Toby values mischief, anarchy, and disorder. Olivia may disapprove of her uncle’s raucous lifestyle, but it is Malvolio whose pompous attitude and sober demeanor contrast with the play’s chaotic, light-hearted tone, and so Sir Toby must triumph over Malvolio to uphold the play’s adherence to merriment. Sir Toby is also seeking a match for Olivia—he makes it clear he’s looking for someone in a comparable social position—which emphasizes the importance of class, and provides further context for Sir Toby’s clash with Malvolio. Malvolio, in Sir Toby’s view, has the audacity to think himself worthy of a countess like his niece, an instance of poor judgment for which Sir Toby and Maria soundly punish him.