A Controller is a Yeerk that is controlling a host, putting it under its total control.
There are Human-Controllers, Hork-Bajir-Controllers, Taxxon-Controllers, Gedd-Controllers, etc. Generally, the suffix is dropped when referring to Controllers of alien races conquered by the Yeerks (Hork-Bajirs, Taxxons, Gedds, etc) because their Controller status is assumed.
When one is infested by a Controller, they have no control over their body and/or any of its actions, though the mind is still alive and aware of everything that's going on, but the Yeerk dominates the host-body, and can access their memories, their privacy, and their secrets and know everything about them, tormenting them and be able to pass through others as though nothing is wrong.
Some people or aliens volunteer and allow themselves to be taken by Controllers, which makes it better for the Yeerks, the host-body is less able to resist the Yeerk inside their heads and can better focus on the task at hand. A particularly cooperative host's mind can even serve as a sort of auxiliary brain, the Yeerk focusing more on motor control and speech, and with the Yeerk taking care of living the host's life the host can focus on correlating information and drawing attention to important things the Yeerk might overlook, giving both a greater capacity for multitasking thoughts and memories. Essentially the Yeerk becomes the driver and his/her host's mind becomes the navigator.
On rare occasions, particularly sympathetic Yeerks end up taking a liking to a particular host, and a strange sort of mutual companionship, even outright friendship, can form. Such controllers are the core of a secret Yeerk Peace Movement, which hold the view that forcibly enslaving hosts is wrong and a controller relationship should be symbiotic and based on mutually beneficial cooperation instead of parasitic total domination. However, due to the Yeerk Military's monopoly on Kandrona Ray access, this movement does not dare preach these views openly for fear of being labeled as traitors and left to starve to death.
Sometimes, when there is some terrible need, the host-mind being suppressed beneath the Yeerk's domination can force its way out and seize control of the body for a few desperate seconds until the Yeerk eventually regains control.
Classes of Host Species
How the Yeerks classify other species.
- Physically unfit for infestation (e.g. the Skrit Na and Hawjabran)
- Capable of infestation, but suffer from severe drawbacks (e.g. the Gedds and Taxxons)
- Physically fit for infestation, but are few in number and can't be bred quickly (e.g. the Hork-Bajirs)
- Perfectly fit for infestation, but are too great a challenge for invasion (e.g. the Andalites)
- Perfectly fit for infestation, are large in numbers and able to breed quickly, and can't resist the Yeerks (e.g. the Humans)
The species used as hosts.
- Arn - Class 1
- Skrit Na - Class 1
- Hawjabran - Class 1
- Gedd - Class 2
- Horse - Class 2
- Taxxon - Class 2
- Ongachic - Class 3
- Hork-Bajir - Class 3
- Leeran - Class 3
- Andalite - Class 4
- Human - Class 4 (Originally Class 5, but after having found humans extremely resilient they were moved to Class 4)
- Nahara - Class unknown
- Mak - Class unknown
- Ssstram - Class unknown
- Garatron - Class unknown