VISSER 3
Visser 3 or Esplin 9466 is the leader of the Yeerk invasion. He's ruthless, power hungry and is scared of only one thing, Jake. He's the only Yeerk to ever capture a Andalite. The Andalite he captured was Alloran-Semitur-Corrass who was a War-Prince. All of the Yeerks underneath his command fear him as when is mad he just lashes out and kills anyone in his reach. He had managed to survive through explosions, battles, whatever is thrown at him (this is very unfortunate!).
Visser 3 was the one who ate Elfangor and he had an ongoing battle with Elfangor over the Time Matrix (for more info about the Time Matrix battle read Andalite Chronicles)
Visser 3 had a variety of morphs and used them. The Animorphs frustrated him very much because the Visser could never beat them.
He is scared or he respects Jake because he is the Animorphs leader and the Yeerks don't exactly know that he is only a teenager who just happened to be in the wrong place at the wrong time.
Close to the end of the entire battle he is promoted to Visser 1.
He is a twin and at the end of the whole entire battle he has trial (the Animorphs available testify) before being placed in jail where he is kept and is never heard of again.