Susan

“Who are you? Where do you fit into poetry and myth? Do you know who I think you are, Ramsay? I think you are Fifth Business. You don't know what that is? Well, in opera in a permanent company of the kind we keep up in Europe you must have a prima donna -- always a soprano, always the heroine, often a fool; and a tenor who always plays the lover to her; and then you must have a contralto, who is a rival to the soprano, or a sorceress or something; and a basso, who is the villain or the rival or whatever threatens the tenor.
"So far, so good. But you cannot make a plot work without another man, and he is usually a baritone, and he is called in the profession Fifth Business, because he is the odd man out, the person who has no opposite of the other sex. And you must have Fifth Business because he is the one who knows the secret of the hero's birth, or comes to the assistance of the heroine when she thinks all is lost, or keeps the hermitess in her cell, or may even be the cause of somebody's death if that is part of the plot. The prima donna and the tenor, the contralto and the basso, get all the best music and do all the spectacular things, but you cannot manage the plot without Fifth Business! It is not spectacular, but it is a good line of work, I can tell you, and those who play it sometimes have a career that outlasts the golden voices. Are you Fifth Business? You had better find out.”
― Robertson Davies, Fifth Business