Animorphs

Everyone thought they had managed to save the governor without any hitches -- except Ax. When he sees Jake, he knows. Jake is one of them, a Controller. The Animorphs now kept Jake tied and up and kept a close watch on him for three days, while Ax becomes Jake for a while, taking his place, being weird at the dinner table, and doing extraordinary well in school. The Yeerk uses all of Jake's memories in a psychological war, preying mostly on Cassie's romantic feelings towards him. If there is a Yeerk in his brain it will die from Kondrona Rays. When Rachel pretended to be asleep, Jake morphed a dog and tried to escape to the nearest Yeerk Pool, but the Animorphs managed to find him, Rachel as a Lion, Marco as a Wolf, Cassie as a Horse, and Ax as Jake. Now the Animorphs know another entrance to the Yeerk Pool. The Yeerk escapes from Jake's body and Jake destroys it. Ax hooked up a system where Jake could call Tom, his brother in another voice, to tell the real Tom that they will figh