In Shakespeare's Twelfth Night, what is the function of Feste in the development of the plot of the play?
Feste in Twelfth Night has a complex role in relation to the development of the story/plot. First, Feste, as the Shakespearean Fool and Olivia's fool/jester, is not involved in the conflict, so his part in developing plot stands in lieu of what the ancient Greek Chorus of Aristotle's era did. Second, Feste is integral in establishing Shakespeare's themes as the plot develops, particularly in relation to Malvolio and the songs in Twelfth Night.
To address the first idea, the Fool in Shakespeare's works in a urban character, as opposed to a rural or country-bumpkin type character, who is foolish, usually with language misappropriations, intentionally as an deliberate act of wit (clowns, the rural country-bumpkins are accidentally amusing and insightful through language confusion and unknowing revelation). In this Shakespearean innovation from Greek comedy, which supplants the Greek Chorus, the Fool acts to narrate through observations and comments on the characters or the progress of events. Therefore, Feste is integral in communicating to the audience deeper knowledge of and editorialized understanding of the characters' feelings and motives and what the audience can expect of them, for instance, saying Orlando's passion is self-indulgence instead of deep sincerity indicating Orlando will be changeable in his affections. Also his name derives from "festival" connecting him with the comedic nature of the play, which might have darker overtones, especially considering Malvolio's treatment, otherwise.
To address the second idea, Feste is the singer of many of the songs in Twelfth Night. One of the most prominent themes that Feste establishes through his songs and in his disguised dealings with the imprisoned Malvolio, is the harsh and destructive effects of time on youth and beauty and love. By this analysis, Feste's dealings with Malvolio are the metaphorical representation of these harsh effects on love, youth and beauty, which makes Malvolio the metaphor for youth, love and beauty. It might seem to be a stretch to analyze Feste's role this way until you realize that Malvolio embodies all the most unstable traits of beautiful youth in love: impressionable, foolish, willing to be self-degrading in order to win the object of their love, absurd, exercising ill judgment, etc. Thus when Toby, Maria and Feste have deferred punishment or no punishment for their behavior, they are extending the metaphorical representation of a foolish beautiful youth in love. Another theme brought out in Feste's songs is the stages of life and the need to enjoy youth while it lasts.