"They needed me to be the bad guy. And I needed them to be the good guys. See, if they were good guys, and I was on their team, then that automatically made me a good guy, too. Even if I was different. At least that’s what I'd been telling myself. Of course, it wasn't quite that simple."
The Yeerks are a parasitic alien species. In their natural state, they resemble large slugs, and live in lakes of sludge known as Yeerk Pools. The Yeerks have the ability to enter another being's body and take over all brain functions, including basic motor and speech control, essentially "becoming" that person. The Yeerks run their civilization as an Empire, ruled by a Council of Thirteen. The Yeerks were discovered by an Andalite named Seerow, who gave them the means to leave their planet; the Yeerks took advantage of his kindness, and used the Andalite technology to invade and conquer other species, for the sole purpose of expansion.