Animorphs Book 22: The Solution by K. A. Applegate

The Solution concludes the David trilogy with a long look at Rachel's role within the team, and how much Jake is willing to use her ruthlessness and Cassie's understanding of people to do a terrible thing.

Rachel is grappling with how Jake sees her and how much he is willing to use her as the ruthless one. Marco is ruthless in an efficient way (this has been discussed previously in the series), but Rachel is ruthless in a reckless way, and this book both shows that and highlights how she can be calculating too.