Visser by K. A. Applegate

Visser by K. A. Applegate has to balance two competing ideas: Hatred for a main enemy of the Animorphs, with some sympathy for her. The explanations are moving and the story is very good but we are not endeared to her. It also sets up the history of the Yeerk invasion on Earth.

It's a very good explanation of how we arrived at this point, but even Visser One's explanation of her own actions are not enough to make her host care about her. Parts of it are things that we might accept from a human as reasons for her actions, but if they were really her motiviation then it wouldn't have made sense for her to call the Yeerk Empire to Earth in the first place. There's something interesting there, both in how the cognitive dissonance is portrayed, and in how each step makes sense with the steps before and after. Even with this, the path of her actions meanders wildly between something understandable to (and even approved of by) her fellow Yeerks, and something that is very, very, human.