Dementia is a nanny who was hired by Cousin Itt and Margaret to take care of their baby, What and soon to be a love interest of Fester.
Upon seeing her come in, Fester is immediately attracted to her, a sentiment which she likewise returns. The two are implied to be similar, most significantly in terms of their physical appearance.
Later, during Gomez's toast at Pubert's party, the two are seen to be close together, happy and smiling at each other, when Gomez toasts "to new friends".
Her name, she states "means insanity".