One role of Feste's is as a stock character. Stock characters were types of characters that Shakespeare used repeatedly throughout his plays, such as parents, clever servants, and of course the clown or the fool. But Feste has a much more significant role than just being the play's fool. For one thing, Twelfth Night was written in honor of the Epiphany celebration held on the twelfth day of December, which marked the arrival of The Magi, or the Three Wise men, who came bearing gifts for baby Jesus. Epiphany was a very wild and festive celebration that involved heavy drinking, revelry, masquerades, and general bawdy foolishness. Feste, whose name contains the root word of festival, embodies the festivities of an Epiphany celebration, particularly, his jokes and singing embody the Epiphany celebration. He sings throughout the play, plus in Act 2, Scene 3, joins in on the loud drinking party both Sirs Toby and Andrew are having by singing them a song, plus joining in on their round-songs.
But Feste is even far more than just an embodiment of festivities. While he participates in the play's merrymaking to some extent, he also remains an observant outsider. In fact, his most important role is as commentator on the other characters' foolish behavior and about the nature of life in general. Feste's role as commentator also more importantly identifies one of the play's most central themes concerning the foolishness of human nature. We see Feste comment on the foolishness of human nature when he calls Olivia's obsessive mourning over her brother foolish. We also see Feste comment on Duke Orsino's obsessive love for Olivia and very keenly observe the truth about Orsino, that he is actually rather fickle and his love for Olivia is insincere. Finally, at the end of the play, we see Feste comment on the hardships of life and how foolishness is linked to those hardships, as we see in his lines:
When that I was and a little tiny boy,
With hey, ho, the wind and the rain,
A foolish thing was but a toy,
For the rain it raineth every day. (V.i.401-04)
Hence, we see that Feste's most central role and purpose is to illustrate the theme of mankind's foolishness.