Antonio plays a small but key function in the plot of Twelfth Night. He is essential to the plot because he is the character who makes Viola suspect that her twin brother Sebastian is probably still alive. Viola is going around disguised as Cesario and in that disguise looks exactly like him. Antonio had developed a deep affection for Sebastian when he found the youth shipwrecked, but Sebastian was alone so Antonio had no clue that he had a twin sister.
Antonio’s role in keeping Sebastian alive also helps propel the action, but his scene with Cesario/Viola is the crucial one. Cesario cannot reveal that “he” is actually Viola because it is too dangerous, and he truthfully denies knowing Antonio, who is in trouble and about to be arrested. Believing that his young friend is denying him, Antonio is deeply wounded. At this point, he speaks bitterly of love’s betrayal:
In nature there’s no blemish but the mind;
None can be call’d deform’d but the unkind:
Virtue is beauty, but the beauteous evil
Are empty trunks o’eflourish’d by the devil.
Meeting next with Orsino, who thinks he acts irrationally, Antonio expands on his complaint against Sebastian, in a substantial monologue. Calling him “most ingrateful,” he reveals that making his presence known in town, where he was in danger of being apprehended on false charges, was all from love:
His life I gave him and did thereto add
My love, without retention or restraint,
All his in dedication
Antonio also helps shape the play’s tone. Because Antonio is a minor character, Shakespeare could easily have given him only brief lines of dialogue with the other, more important characters. Instead he chose to give Antonio a substantial monologue. In addition, this overt profession of love by one man for another in front of many witnesses is different in tone from the typical jesting and bantering. These choices indicate additional, more substantive functions for this character, and thus Antonio brings some gravity to the play. Speaking of love for another man also adds a homoerotic dimension, which has been much commented on, especially in modern times.
Overall, Twelfth Night is a light comedy with a lot of foolishness revolving around mistaken identity, love-sick people chasing each other, and similar stock features of Elizabethan comedy. The dark moments revolve around the torment and false imprisonment of Malvolio. The important moral characteristics of loyalty and honesty are essential parts of Antonio’s nature. While Viola’s disguise protected her, she has definitely been having fun with the liberty that external male appearance allows her. Even though she honestly does not know Antonio, she must confront the pain and anger he expresses.