In Shakespeare's 'Twelfth Night', what is the name of Sebastian's twin sister?

Viola is the heroine and protagonist of the play Twelfth Night, written by William Shakespeare.

Viola's actions produce all of the play's momentum. She is a young woman of Messaline, a fictional country invented by Shakespeare. In the beginning Viola is found shipwrecked on the shores of Illyria and separated from her twin brother Sebastian. Not knowing whether Sebastian is alive or dead, the Sea Captain that tells her that this place is ruled by the Duke Orsino, who is in love with the Countess Olivia. Viola wants to serve her, but finding this impossible, she has the Sea Captain dress her up like a eunuch, so she can serve the Duke instead.