Yeerks

At first, when the Animorphs first discovered the invasion their planet faced and received their morphing power, they knew little about the Yeerks. They knew only that they were slugs who crawled into people's ears and took over them, making them Controllers. They were the enemy, and they were to be killed, that was enough. But gradually, as the battles went on, they found out more. The Yeerks, even though they are mainly charactarized by their ruthlesness and desire to conquer the galaxy, and by their thievery of a sentient being's physical freedom because they have none of their own, are a people in their own right. Soon the Animorphs see that they are not just some faceless enemy; they are are a thinking race made up of individuals, each with their own thoughts and dreams and ambitions, and their own purposes in life, good or evil. Though all too often, whether because they are simply conforming to the society and the system set up hundreds of generations before them, or if they are scheming and killing to climb higher in the hierarchy, they have often only served to enslave people, entire races, and destroying all worlds they lay their hands on to make them over again to resemble their own.

There is hope for them, though. At the current time, the peaceful Yeerks, those such as Aftran 942, who are looking for a better way are forced into silence and submission by their superiors and most of the masses. Those who would speak out, if not killed outright for treachery, would find themselves falling victim to some sort of accident shortly after making their opinion public. However, it is inevitable, that, if not sometime soon, at least not too far into the future that in some way or another, the war that started with the Yeerks and the Andalites, has to end. When it does, their fate will be uncertain, depending on how the Andalites and possibly the humans choose to deal with them, but, no longer being at war, those who wish to be at peace and equality with the other races, not to enslave any unwilling hosts, though how that would be accomplished is vague at this time, those Yeerks would have the chance to gain the upper hand, and could possibly change their society around.

Here I have various information things I have written or collected from the Animorphs books:

Known Yeerks: Listed by rank
Article: Yeerks in General and Their Psychology
Yeerk Biology: Life-cycles, natural bodies, etc.


Animorphs Encyclepedia
The Vortex