Illim was the Yeerk who controlled Mr.Tidwell, the teacher at the Animorphs' school and together they were members of the Yeerk Peace Movement. Illim began as a regular type of controller but then was moved by his host's cries and protests. Illim and Mr.Tidwell conveyed to Cassie the message that Aftran had been captured. Later, Cassie morphed Illim so she could enter the Yeerk Pool inside Mr.Tidwell.