In "Twelfth Night," who is Viola's twin brother?

In this play, Viola has a twin brother whose name is Sebastian.  Her search for him leads to much of the action in the play.

We first meet Viola in Act I, Scene 2.  She has been shipwrecked and is talking to the captain of the ship.  She had been on the ship with her brother, Sebastian, but he is nowhere to be found.  Because of that, Viola believes that he has been drowned.

However, she later hears that he may not have died.  So she dresses up like a boy and goes looking for him.