A Sea Captain was the captain of the ship that Viola and Sebastian were traveling on.
He saw Sebastian tie himself to a mast, and so offers Viola some hope that he may still be alive. He tells Viola of Illyria, Orsino and his love for Olivia. He accepts to help Viola disguise herself as a man and to present her to the Duke, and keeps her women’s clothes in his custody. He is later arrested at Malvolio’s suit, but Olivia promises that he will be freed and able to tell his part in the story.