what is Olivia's dramatic purpose in twelfth night?

A noblewoman, Countess of Illyria. She lost her father, then her brother right before the beginning of the play; she resolves to be in mourning for her brother for seven years, thus thwarting Orsino's attempts to woo her. She becomes enamoured of Viola, who acts as a messenger for Orsino; and though Olivia tries her best to win Viola, Viola does her best to let her down easily, because Viola is not a boy like she pretends to be. Olivia rushes off and marries Sebastian, thinking that he is Viola as Cesario; but everything works out in the end.