who is fifth business? according to whom?

Liesl first quotes the term "Fifth Business" to Dunstan, though the reader knows the phrase from the inscription of the novel's first page. A dramatic type that was meant in certain periods of theatre to serve as the confidant character, with little direct influence on the plot, the "Fifth Business" is destined to remain in the background. In many ways, Dunstan's role as "Fifth Business" is what endears him to us. As an everyman protagonist who is simply in position to report on extraordinary events, we get to experience his singular life as he does. And yet Dunstan ultimately transcends this role, suggesting that the title is supposed to be somewhat ironic. This irony helps establish one of the story's ultimate points: we ought to embrace our identities so that we can then work to transcend and grow past them.