Sebastian is Viola’s twin brother who does not merely look like her but also possesses affection, bravery, and self-assertion just like her.
However, his appearance in the play is limited, we get to know about him throughout the play with Viola’s character before it is finally confirmed to us when he comes that is exactly like her.
His introduction in the play, through the words of the sea captain, emphasizes his courage and self-reliance, “I saw your brother, most provident in peril, bind himself, courage and hope”.
It is Sebastian’s affection that justifies the deep love and loyalty that Antonio feels for him in such a short time. The depth of Antonio’s feelings is realized in his plea, ‘If you will not murder me for my love, let me be your servant”.
Shakespeare makes us realize that Viola’s love for him is also possible because of Sebastian being a compassionate brother to her.
It is not because of Olivia’s love for Viola that lets her fall for Sebastian since she mistakes him for Cesario, rather it is possible because he holds his own individuality. He is kind, vocal and knows himself well.
“had it been the brother of my blood, I must have done no less with wit and safety”, he says to Olivia. Sebastian is a man of action, unlike Duke Orsino who is a man of words. He acts to solve challenges that come into his way.
He accepts to fight with Sir Andrew and Sir Toby but he does not like promoting violence. His idea of manliness does not come from a conventional idea. He does not assert himself with the use of his muscles but with the use of his generosity. “I am sorry, madam, I have hurt your kinsman“.
Although we see very little of Sebastian in the play, the romantic comedy would have been incomplete without him. It is only because of him that we get to know about Shakespeare’s idea of an ideal lover.
He may have accepted Olivia’s advances towards him instantly, but his response to her is that of commitment and loyalty. His decision is also based on his quick and true assessment of Olivia’s character.
We know that Sebastian is not unconscious while committing to Olivia because we have seen him as loving and genuine as a brother, friend and even as a (minor) rival.
The sense of betrayal in Antonio is much greater when he confuses Viola to be Sebastian because the warmth and love of Sebastian for him have been of the same intensity. The confusion which could have escalated otherwise gets solved only due to the character of Sebastian.