In what ways is Viola loyal?
Viola's loyalty is tremendous, I think, to Orsino. She is, from very early in their relationship, madly in love with him, but she chooses not to say anything - and she still agrees to carry out her duties of pleading his love for another woman, Olivia.
And, she does such a good job in pleading Orsino's love that Olivia falls in love with her, thinking her a boy (as per her disguise).
Viola's loyalty extends beyond mere professional loyalty, though. In the unusual last scene of "Twelfth Night", Orsino, maddened with love for Olivia, and furious that Olivia is in love with "Cesario" (really Viola), announces that he's going to murder Cesario so that Olivia can't have what he wants. And look at Viola's response:
Come, boy, with me; my thoughts are ripe in mischief:
I'll sacrifice the lamb that I do love,
To spite a raven's heart within a dove.
And I, most jocund, apt, and willingly,
To do you rest, a thousand deaths would die.
So desperate is she to free Orsino from his unhappiness, Viola would even give up her life. That's loyalty - beyond loyalty!