Viola
The protagonist. She survives a shipwreck, invents a male identity, and spends the play trapped between the role she created and the feelings she cannot express. She is the most capable person in the play and the most constrained by circumstance.
Orsino
The Duke of Illyria. He is in love with Olivia, or more precisely with the experience of longing for her. He treats Cesario as a confidant and is the last person to figure out what has been happening around him.
Olivia
A countess in mourning who refuses Orsino's advances and then immediately falls for his messenger. She is decisive and self-possessed, but her feelings are directed at a fiction. She gets a real husband in the end, just not the one she thought she was marrying.
Malvolio
Olivia's steward. He is self-righteous, ambitious, and easy to manipulate because he wants to believe the best about himself. The prank destroys him socially and he leaves the play without reconciliation, which makes him the most tragic figure in a comedy.
Sebastian
Viola's twin brother, presumed dead. He arrives in Illyria and is immediately mistaken for Cesario. He agrees to marry Olivia within minutes of meeting her. His presence is what makes the resolution possible.
Sir Toby Belch
Olivia's uncle and the engine of the subplot. He drinks, schemes, and uses Sir Andrew's money and affection for his own entertainment. He orchestrates the prank on Malvolio and marries Maria at the end, which is the closest thing he gets to a consequence.
Maria
Olivia's waiting woman and the actual architect of the Malvolio prank. She is clever, loyal to Sir Toby, and the most competent schemer in the play. She marries Sir Toby as a reward, though the play treats this as an afterthought.
Feste
The fool who serves both Olivia and Orsino. He is the sharpest observer in the play and the one character who is never fooled by anyone. His closing song steps outside the comedy and reminds the audience that the world keeps going after the party ends.
Sir Andrew Aguecheek
Sir Toby's companion and Olivia's hapless suitor. He is vain, cowardly, and completely manipulated by Sir Toby. He ends the play beaten and abandoned, the clearest victim of the subplot's cruelty.
Antonio
The sea captain who follows Sebastian out of devotion. He risks arrest to stay close to him and ends up betrayed by a case of mistaken identity. His loyalty is genuine and his reward is minimal.