As the play progresses, she proves herself to be the decent person as promised in this first encountering. We soon learn that she is intelligent, witty and charming, and there are many examples of occasions illustrating this, for example during her short conversation with Feste in Act 3, Scene 1:
VIOLA: Save thee, friend, and thy music! Dost thou live by thy tabor?
FESTE: No, sir, I live by the church.
VIOLA: Art thou a churchman?
FESTE: No such matter, sir. I do live by the church; for I do live at my house, and my house doth stand by the church.
VIOLA: So thou mayst say the king lies by a beggar, if a beggar dwell near
him or the church stands by thy tabor if thy tabor stand by the church.
FESTE: You have said, sir!
It is these qualities that enable Viola to gain Orsino's special confidence and that cause Olivia to all in love with her. Her conversations with Orsino and Olivia show that she is a straightforward and honest character in spite of the deception she is forced to enact for her own survival; she loyally continues to thy to win Olivia's love for Orsino, even though she loves him herself and she treats Olivia with dignity when she confesses her love for her (as Cesario). Such capacity for deep feeling is something which is most prominent throughout the play, in particular in Act 2, Scene 4, in which she tells Orsino a story, which begins, "My father had a daughter lov'd a man…"
Olivia, however, possesses a number of altogether characteristics. Our first encounter with her is certainly less favourable than that of Viola. Having made an extravagant vow to mourn her brother for seven years, we soon witness her breaking her promise, and thus her capability of self-deception. So, although it could be argued that Viola and Olivia are similar in that they both adopt a form of disguise, the fact that Olivia, in her melancholy mood, makes and breaks this vow, which, since it seems to mean little to her, has no real purpose, whereas Viola disguises herself as a means of survival, illustrates that in fact they are not.
However, as the play progresses, a great deal more of her favourable characteristics become apparent. She, like Viola, is intelligent and proves herself to be adaptable in her dealings with other members of the household - she soon allows Feste to win her over with his clowning, and her characterisation of Malvolio in Act 1, Scene 5 is extremely accurate:
OLIVIA: O you are sick of self-love, Malvolio, and taste with a
distempered appetite. To be generous, guiltless, and of free disposition is to take those things for bird-bolts that you deem cannon bullets. There is no slander in an allowed fool though he do nothing but rail; nor no railing is a known discreet man though ho do nothing but reprove."
However, the quality which most characterises Olivia is something which contrasts a great deal with the strong sense of practically possessed by Viola: her capacity for impetuous feeling. Olivia pursues Cesario relentlessly, with little concern for the behaviour demanded by her own status as a countess and "his" as a servant. And with the passion comes a sense of sentimentality, not only reflected in her behaviour towards her Viola, but many other characters too, even Malvolio, who is undoubtedly the character with who we least sympathise, and when Malvolio claims, "I'll be revenged on the whole pack of you", her compassionate nature causes her to respond, "He hath been most notoriously abused."
So then, to conclude, Shakespeare undoubtedly intended the audience to draw some parallels and recognise a number of similarities between the characters Viola and Olivia, which is suggested by the similarity of their names, their situation and their link with Orsino. However, this does not mean to say they are necessarily similar in character - as illustrated above they are clearly not. Viola is, amongst many things, practical, sensible, sympathetic and very much in control. In complete contrast, Olivia is emotional, sentimental and changeable. However, this does not mean to say that one character is better or worse than the other - though throughout the play Shakespeare makes it very clear that they posses different traits, ultimately they are "good" people, and this "finished product" is a great deal more important than the route taken by each in order to achieve this.