Fertle is a flaybooz who is married to Random Dent in the world the Guide Mark II lets her live in her head. Her mother, Trillian, is very annoyed that Random has married Fertle, as flaybooz are essentially small rodents.
Although he is intelligent and has a mental bond with Random, he cannot speak - except when he said the word "Battery" as the Guide's warning that Random's alternate world was running out of power.