The Duchess of Nuts is a creepy, mystical character who first appears in “The Duke of Nuts”. She, hence the name Duchess of Nuts, is the Duke of Nuts’ wife and Marquis of Nuts’ mum.
When Finn and Jake enter the Nut Duchy, she invites them in and tells them that “the nuts told her of their coming”. Like the Duke of Nuts, she also appears in “Henchman” as a more social character.