The Old Woman in the Cave is a character who appears in Fit the Twenty-Fourth of the Quintessential Phase and the novel Mostly Harmless and is played by Miriam Margolyes.
She lives in a cave on the planet Hawalius, where she spent most of her time fending off outsized flies from a smelly dead goat. She herself was also very smelly.
Sometimes visitors, such as Arthur Dent would ask her for advice. When this happened, she hauled a photocopier out of the cave, and copied her life story. She then advised them to do the complete opposite of anything she had done, on the grounds that this would probably help them avoid ending up in a smelly old cave at the end of their lives.