Feste

A "fool" was a character in William Shakespeare's play Twelfth Night.

After outlining the play and its characters in a meeting with Richard Burbage and the Fifth Doctor, Burbage added "And a fool. There must be a fool". Shakespeare agreed, stating nothing but "Assuredly" in response. (PROSE: Diary Extract)

Behind the scenes
Although not named in Diary Extract, the character is a clear reference to Feste.