Fabian is a member of Olivia’s household who has lost that lady’s favor due to Malvolio’s telling her about a bear-baiting Fabian was involved with.
For revenge, he joins in the plot to make a fool of the steward. He also helps Sir Toby keep control of Sir Andrew. Apparently a natural prankster, he helps to set up the duel between Andrew and Cesario, building up the latter’s fear of Sir Andrew. Feste does not appear to consider him trustworthy, refusing to let him see the letter Malvolio sends to Olivia from his prison. Having later read the letter aloud to Olivia and all the others, Fabian quickly confesses the whole plot, in the hope that with everyone being so happy and about to be married, they won’t be too hard on the plotters, and take it all as a good joke.