"Twelfth Night" is a comedy about appearance and perception and the path to true love. The theme of love is predominant but with acquiring love has some caveats, as explained here at eNotes:
"Love is true in Twelfth Night, but it is also irrational, excessive and fickle; it wanes over time, as does its chief cause, physical beauty. Duke Orsino opens the play with the lines: If music be the food of love, play on, / Give me excess of it; that surfeiting, / The appetite may sicken and so die (I.i.1-4).
The main characters are:
Orsino—the Duke of Illyria, who is madly in love with Olivia.
Olivia—the countess with whom Orsino is in love and who rejects him.
Viola—the female of a brother–sister pair of twins who enters Illyria disguised as Cesario and finds love.
Sir Toby Belch—Olivia’s uncle who drinks a lot and marries Maria.
Malvolio—steward to Olivia who fancies himself in love with her.
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You need to phrase your question a little better in order for us to give you a meaningful answer. "Characterization" is the way in which the playwright reveals the personalities of the people in the play. Are you asking for a description of the different characters? If so, you can find a good summary through eNotes.