Viola also has natural intelligence, wit, and a vast amount of charm. These qualities will help her obtain her position with Duke Orsino, and they are also the same qualities which cause Lady Olivia to immediately fall in love with her male disguise, Cesario. It is her charming personality, we should remember, which won her the sea captain's loyalty, who helped her disguise herself and which without him would have never succeeded.
Within a short three days' her wit, charm, loyalty, and her skill in music and conversation won for her the complete trust of Duke Orsino. We should also remember that even though she is in love with the duke, she is loyal in her missions when she tries to win Lady Olivia's love for him.
For the modern audience, Viola's charm lies in her simple, straightforward, good-humored personality. She could have used her disguise for all sorts of connivings, yet she is honest in all of her dealings with Lady Olivia and with Duke Orsino, all be it she does use her disguise to entertain the audience with her puns and when talking to Feste the Clown. Perhaps the most surprising thing about Viola is that a young lady in possession of so many attributes falls in love with someone who is as moody and changeable as the duke.
Sebastian is Viola's twin brother who has been lost at sea. He is separated from Viola but survives the ship wreck by clinging to the ship's mast. Eventually, he's fished out of the ocean by a sailor, Antonio, who falls in love with him.
We don't see much of Sebastian in the play, but his character is significant in the play. Once Sebastian travels to Illyria, he's mistaken by all for "Cesario" and quickly agrees to marry Olivia. Twelfth Night is a comedy and, as such, it works its way toward marriage and the reunification of families.That said, Sebastian clearly has a close relationship with Antonio, who may or may not be a lover. In this way, Sebastian's relationship allows the play to study, briefly, the erotics of male bonds.
We can see some of the differenced in Viola’s and Sebastian’s character, especially when Sebastian is confused with Cesario when Sir Toby and Sir Andrew are looking for a fight. Where Sebastian fought back, we can clearly see that Cesario/Viola would have fled to safety. This is due to the fact that she is a woman so her fighting skills are not up for the challenge. We also see differences in the fact that Viola is mature and responsible whereas Sebastian is wreak less, this can be seen when Olivia asks him to “be ruled by” her, and even after just several minutes of knowing who she is he agrees – “Madam, I will”. Furthermore we can see the differences between brother and sister because Viola is a wittier, more ‘on the ball’, person whereas Sebastian has more of an easy going character.