Feste in particular is known to have a very significant role in Twelfth Night. Not only does his intellectual wit and word play amuse, he reveals great many truths about the other characters and about reality in general. Even though Feste plays the role of a fool, or court jester, Feste proves that all of the other characters are foolish except for him.
One example of Feste proving other characters to be foolish can be seen in the very first scene in which we meet him. In Act 1, Scene 5, Maria opens the scene by warning Feste that Olivia will "hang thee for thy absence," and Maria is proved correct when Olivia's first response at seeing Feste is to command that he be taken away (I.v.3). Always the wit, Feste turns her words around by commanding that Olivia be taken away as being the "fool" rather than he. He then commences to prove she is a fool by asking her why she mourns her brother's death and stating that her brother's soul must be in hell. When Olivia replies, "I know his soul is in heaven, fool," Feste proves her prolonged mourning is foolish by stating, "The more fool, madonna, to mourn for your brother's soul being in heaven. Take away the fool, gentlemen" (63, 64-65). In other words, Feste is rightfully stating in his comic way the truth that Olivia's insistence at shutting herself out from society and continuing to prolong her grief is an absolutely foolish activity. Later, Feste proves Malvolio to be foolish with other twists of words.
Beyond proving other characters to be foolish rather than himself, he is also known to make very profound and theme-revealing statements about the reality of love. One of Shakespeare's main points is to show the foolishness, fickleness, and arbitrariness of love. Feste asserts the foolishness of love in the lyrics he sings to Sir Toby and Sir Andrew, "Journey's end in lovers meeting, / Every wise man's son doth know" (II.iii.41-42). What he's saying here is that lovers' journeys end when they finally meet each other, which is a sweet statement about love, but his most telling line is the last line of the song. If the wise man's son knows that lovers' journeys end when lovers meet, then it's not the wise man who is experienced with love, but rather the wise man's son--the foolish son, showing us the point that love makes lovers foolish.
Hence we see that while Feste is a comic character, he actually serves as the play's most enlightening figure. He not only reveals the play's themes, but reveals certain truths about the characters and the nature of love.