After the destruction of the Pemalites at the hands of the Howlers, Erek, along with the rest of the Chee, escaped to Earth, and has lived there for thousands of years, using incredibly advanced holographic technology to pass as human. In an early book, Erek, through the use of the Pemalite Crystal, reprogrammed himself to remove his inability to do violence and slaughtered about two dozen Hork-Bajir, but, unable to cope with the memory of violence and unable to forget it, he vowed never to fight again. The Pemalite Crystal was discarded (see Chee). The Animorphs befriended Erek after learning he was a spy against the Yeerks, and Erek - and most of the other Chee - assisted them whenever they could, whether it was passing them sensitive Yeerk information, masquerading as the human Animorphs while the real Animorphs were away on a mission, or even assisting the Animorphs on some of the missions (such as #26: The Attack).
As the series progressed Erek became a more important character, until the final battle in #53: The Answer and #54: The Beginning, in which the Animorphs forced Erek's hand, taking advantage of his non-violent programming to force him into acting in situations that he felt were morally reprehensible. After the defeat of the Yeerk Empire, Cassie told Erek that Jake said for him to leave the ship if Erek wanted to keep the Chee identity secret since the Andalites were coming on board. This was not a friendly parting since Jake (and maybe others) blamed Erek for the death of Rachel since Erek drained the Dracon beam so Jake would not be able to defend the ship and possibly kill others, while Erek was horrified by Jake's decision to vent the Yeerk pool, killing those yeerks feeding therein. The Blade Ship manned by Jake's brother Tom was going to fire on the Pool Ship thus Rachel had to attack Tom to stop it. Both ended up dying. Erek left telling Cassie to take care of Jake since Jake would need her. The series does not mention what happened to him or the other Chee after the battle's end.