Essak only appears in The Familiar. In the future shown in The Familiar, all of the Animorphs except for Rachel and Tobias are made into Controllers. Jake's Yeerk is known as Essak 2041275. Even though he exists in this strange future, Essak is not in Jake's head. Essak was a scientist. He was working on the moon ray project, the goal of which was to turn Earth's moon into a Kandrona. He was close to Marco's yeerk, Visser four. Jake constantly doubts that the yeerk exists. Due to the bizzare nature of the book, there is a good chance Essak dosen't even exist.