Human Controller

In the Animorphs series of children's books, a human controller is a human who has a Yeerk (an alien parasite) residing in their brain and controlling them. The Yeerk's control of the person is unnoticeable by other people; once the Yeerk enters the head through the ear canal and wraps itself around the brain, it controls the emotions, thoughts, memories and attitude so well that it is like nothing ever happened.

Under extreme emotions, the human might regain control of the body for a short time, as shown in book #2, The Visitor.

There are only two known weaknesses of the control a Yeerk exerts over a human. The first is kandrona deprivation: once the Yeerk has stayed in the host for three days it must leave the host to go to the Yeerk pool containing kandrona rays which Yeerks need to survive, thereby giving its host temporary freedom. The hosts are kept in cages for this time to await re-entry by the Yeerks. The second weakness is their vulnerability to maple-flavoured instant oats, which acts as a drug to the yeerk, driving it crazy and returning partial control to the human, although it also keeps the yeerk alive and makes it impossible to remove it from the brain.