The Fool was a plot device in a campaign I ran for the party. Generally, he was an immortal with whatever skills and powers I needed him to have at the time.
He would pop in every now and then and drop clues, hints, or sledghammers when needed.
Funny thing is, he worked. They were never quite sure what he was up to when he came by, but were always interested in what he had to say. Funny thing is, they could never get him to tell the same story twice without changing something. Seems he was a bit of a liar.
Since the only name that anyone had for him was 'The Fool', and he never gave them any other name, they eventually named him themselves: Jack (of fools).
Any other GM have a bunch of character's always keep a place at the table setup in case an NPC showed up? All the time?
Later,
John Bowden