Viola - A young woman of aristocratic birth (rich, ruling class), and the play’s protagonist. Washed up on the shore of Illyria when her ship is wrecked in a storm, Viola decides to make her own way in the world. She disguises herself as a young man, calling herself "Cesario," and becomes a page to Duke Orsino. She ends up falling in love with Orsino—even as Olivia, the woman Orsino is courting, falls in love with Cesario. Thus, Viola finds that her clever disguise has entrapped her: she cannot tell Orsino that she loves him, and she cannot tell Olivia why she, as Cesario, cannot love her. This is the central conflict in the play.
Orsino - A powerful nobleman in the country of Illyria. Orsino is lovesick for the beautiful Lady Olivia, but becomes more and more fond of his handsome new page boy, Cesario, who is actually a woman—Viola. Orsino is a vehicle through which the play explores the absurdity of love: a supreme egotist (full of himself), Orsino mopes around complaining how heartsick he is over Olivia, when it is clear that he is chiefly in love with the idea of being in love and enjoys making a spectacle of himself.
Olivia - A wealthy, beautiful, and noble Illyrian lady, Olivia is courted by Orsino and Sir Andrew Aguecheek, but to each of them she insists that she is in mourning for her brother, who has recently died, and will not marry for seven years. She and Orsino are similar characters in that each seems to enjoy wallowing in his or her own misery. Viola’s arrival in the masculine disguise of Cesario enables Olivia to break free of her self pity. Olivia seems to have no difficulty transferring her affections from one love interest to the next, however, suggesting that her romantic feelings—like most emotions in the play—do not run deep.
Sebastian - Viola’s lost twin brother. When he arrives in Illyria, traveling with Antonio, his close friend and protector, Sebastian discovers that many people think that they know him. Furthermore, the beautiful Lady Olivia, whom he has never met, wants to marry him. Sebastian is not as well rounded a character as his sister. He seems to exist to take on the role that Viola fills while disguised as Cesario—namely, the mate for Olivia.
Malvolio - The straitlaced steward—or head servant—in the household of Lady Olivia. Malvolio is very efficient but also very self-righteous, and he has a poor opinion of drinking, singing, and fun. His "better than everybody else attitude" annoys Sir Toby, Sir Andrew, and Maria, who play a cruel trick on him, making him believe that Olivia is in love with him. In his fantasies about marrying his mistress, he reveals a powerful ambition to rise above his social class.
Feste - The clown, or fool, of Olivia’s household, Feste moves between Olivia’s and Orsino’s homes. He earns his living by making pointed jokes, singing old songs, being generally witty, and offering good advice cloaked under a layer of foolishness. In spite of being a professional fool, Feste often seems the wisest character in the play.
Sir Andrew Aguecheek - A friend of Sir Toby’s. Sir Andrew Aguecheek attempts to court Olivia, but he doesn’t stand a chance. He thinks that he is witty, brave, young, and good at languages and dancing, but he is actually an idiot.
Antonio - A man who rescues Sebastian after his shipwreck. Antonio has become very fond of Sebastian, caring for him, accompanying him to Illyria, and giving him money—all because of a love so strong that it seems to be romantic in nature. Antonio’s attraction to Sebastian, however, never bears fruit.
(from Spark Notes)
Surfeit
A lot of something, excessive
Abatement
the act of making less active or intense
Fancy
not plain; decorative or ornamented
Pestilence
any epidemic disease with a high death rate
Brine
a strong solution of salt and water used for pickling
Sovereign
a nation's ruler usually by hereditary right
Provident
giving something useful for the future
Prattle
speak about unimportant matters rapidly and incessantly
Virtuous
morally excellent
Virtue
the quality of doing what is right
Eunuch
a man who has been castrated and is unable to reproduce
Prodigal
recklessly wasteful
Quarreler
Someone who argues
Accost
approach and speak to someone aggressively
Constellation
a configuration of stars as seen from the earth
Strife
bitter conflict; heated often violent dissension
Transgress
act in disregard of laws, rules, contracts, or promises
Amends
something done or paid to make up for a wrong
Infirmity
the state of being weak in health or body
Folly
the trait of acting stupidly or rashly
Lethargy
inactivity; showing an unusual lack of energy
Reprove
reprimand, scold, or express dissatisfaction with
Feigned
not genuine, being fake
Mollification
the act of appeasing someone
Homage
Giving respect to something/someone
Profanation
bad behavior
Recompense
make payment to
Nonpareil
model of excellence or perfection of a kind
Gracious
characterized by kindness and warm courtesy
Canton
a city on the Zhu Jiang delta in southern China
Contemn
look down on someone
Reverberate
ring or echo with sound
Contempt
lack of respect accompanied by a feeling of intense dislike
Peevish
easily irritated or annoyed
Distemper
any of various infectious viral diseases of animals
Extort
obtain by coercion or intimidation
Mirth
great joy and happiness
Mitigation
the action of lessening in severity or intensity
Uncivil
lacking civility or good manners
Indignation
a feeling of righteous anger