Nearly all of the characters stand out as very vivid, developed characters, but one of the vaguest characters in Twelfth Night is Fabian. Literary critics agree that he is a vague character because he seems to appear out of nowhere in Act 2, Scene 5 and also, more surprisingly, to take the place of Feste in this particular scene. What's more critics points out that we are told very little about his character. We are not even told what his role in Olivia's household is. All we know is that he also holds a grudge against Malvolio ("Feste and Fabian: Plots and Complots").
We know that Fabian took the place of Feste in Act 2, Scene 5 because when Maria lays out her plan, she suggests that both Sir Andrew and Sir Toby will witness Malvolio's reactions to the love letters and that Feste will make a third witness, as we see in her line, "I will plant you two, and let the fool make a third, where he shall find the letter" (II.iii.158-59). Critics argue that since Feste remains silent in the rest of this scene, his silence is a way of refusing to join in on the scheme because Feste's role in the play is as a "commentator and analyst [who] ... provides the link between the world of the play and the audience's world in the 'wind and rain' of reality" ("Feste and Fabian"). Hence Fabian takes Feste's place as their third witness to the antics surrounding Malvolio because Fabian is just as foolish as the rest of the characters, while Feste is above the foolishness.
Later, when Fabian does make his first appearance at Sir Toby's invitation to witness the joke, all we learn about him is that he feels just as insulted by Malvolio as the other characters. He especially wants revenge on Malvolio because, "[H]e brought me out o' favour with my lady about a bear-baiting here," which apparently means that Malvolio tattled on him to Olivia about his having organized a dog fight in which dogs attack a captured bear (II.v.6-7). But other than knowing that he is very happy to be joining in on the fun of mocking Malvolio, we learn nothing about his character; we especially do not even learn what kind of servant he is. Plus he is only present in a few different scenes, all of which have to do with the foolish antics of Sir Toby and Sir Andrew.