The fool is the only one of the major arcana in a tarot deck without a number.
"Beware, Sir Gawain: if the lance catches your backside,the ladies in the tower shall have another moon at which to stare."
Certainly the fool is a simpleton. Maybe he has been made simple by love, like Sir Gawain. Or maybe he is just simple-minded by nature:
"Lament, Parsifal the wretched: through your simple-mindedness,the Fisher King yet suffers from his wound."