Who is Sebastian in the play Twelfth Night?

In the play Twelfth Night by William Shakespeare, Sebastian is a young nobleman, the twin brother of Viola. Sebastian is separated from his sister during a shipwreck at the beginning of the play and believes that she has died. He arrives in Illyria, where Viola is disguised as a man named Cesario, and falls in love with Olivia, the Countess of Illyria. Sebastian's presence in Illyria ultimately leads to the revelation of Viola's true identity and the conclusion of the play.