*All quotes are taken from the Norton Shakespeare, based on the Oxford Edition.
Viola's relationship with the Captain is that of a confidant. The Captain is there when Viola arrives ashore in Illyria. She has lost sight of her brother and fears that he has drowned. The Captain tries to ease Viola's mind by telling her that he saw her brother: "...Most provident in peril, bind himself...To a strong mast that lived upon the sea..." (Lines 11-13) He gives Viola hope. The Captain also provides Viola with information regarding Duke Orsino. Viola confides in the Captain about her plans to disguise herself and become the duke's servant.
Duke Orsino is very egotistical and greedy. He has grave misconceptions of reality as well as the value of true human emotion. He is extremely superficial and does not show any regard for the world around him, as he is too focused on his own life.
Because a duke has duties and responsibilities that affect other people, it is not appropriate for a man of his rank to behave this way. However, the plot would not be successful without Orsino's superficiality.