First, stock characters are loosely defined as characters who reappear in literature and folk tradition and represent types of people who seem to turn up in everyone's life at some point--for example, the wicked stepmother; a damsel in distress; a powerful, corrupt businessman or politician; a friend who means well but is a fool; a person who poses as a friend but who tries to destroy his friends. In many cases, stock characters simply have a function, that is, a doctor shows up to pronounce someone dead or a priest arrives to marry two lovers. And most stock characters have names that disclose either their nature or the role in the play or both.
In Twelfth Night, some stock characters are necessary for the plot to move forward. For example, Malvolio, whose name means evil will in Latin, is described as a "kind of Puritan," which immediately places him in the persona non grata category for Elizabethans like Shakespeare who view Puritans as mis-guided religious heretics whose natures are perverted by their repressive beliefs. As his name suggests, Malvolio wishes ill for everyone, except himself, and he becomes a subject of ridicule because, on one hand, he behaves as a strict Puritan, and, on the other hand, he dreams of marrying Olivia and enjoying her wealth.
Sir Toby Belch, as his name implies, has some social stature but is generally disgusting because all his efforts are centered on eating, drinking, carousing and, true to his name, belching. Interestingly, Sir Toby embodies the festival atmosphere of Twelfth Night (January 6), which signals the end of a Christian religious festival, and so Sir Toby serves as both a stock character--the out-of-control but good-natured relative--and the symbol of the Twelfth Night celebration.
Sir Toby's friend, Sir Andrew Aguecheek, is another great stock figure because he is, like Sir Toby, is a great drinker and carouser and therefore central to the Twelfth Night celebration. Sir Andrew, however, also has pretensions for Olivia's hand in marriage and is convinced by Sir Toby to challenge Cesario (Olivia in disguise), which comes to nothing when Antonio intervenes. Sir Aguecheek is, as his name suggests, not up to any kind of duel or battle--his name, which includes the word ague, commonly used to refer to any kind of fever or wasting illness in Elizabethan times, marks him as physically unfit for a marriage with Olivia. He represents the stock figure of an unsuitable marriage match for both moral and physical reasons.