I think it is so important that the last book in the three part David arc in animorphs is narrated by Rachel.
Marco notices the issues with David right away and distrusts him, but the early red flags are pushed down.
In book two of the arc, Jake must make difficult decisions as a leader and deal with the danger in David's wake. When he thinks Tobias was killed, he tells ax to get Rachel. Not cassie. Not Marco. Rachel is the one he needs, the one who will do what must be done.
David is a sociopath who has no reservations about killing. He tries to defect to Visser Three's side. He kills animals without remorse. Morphs humans, including Jake and Rachel's dying cousin to take his place.
The Solution is the final book in the arc, and there is a lot of talk about Rachel being the one who does the dirty work. Jake tries to deny it at first, but then is honest with her that he needs her for the difficult jobs no one else will or can do. This arc shows Jake as a leader who understands his team, specifically pointing out he doesnt understand David. He understands Rachel, knows there is some darkness in there that is unfortunately very useful to the war effort.
I think it is important that Cassie is the one who knows how to manipulate David and trick him into becoming a rat nothlit. I think it is important that Cassie knows using Rachel, letting David think he has power over her, is the best way to trick David.
The animorphs know their roles. Books 14-17 or so are all about their roles in the team and the way they are being constricted by their roles. The leader, the funny one, the brave one, the moral one. They know their abilities. And at this point in the series, K A Applegate is showing how they use each other, trust each other to use them. They are friends. They are soldiers. They are family.
Rachel talks about not recognizing herself any more. She used to be a mallrat, a gymnast, a big sister. Now she is a lethal and reckless soldier. But she still has those pieces of herself, somewhere. She uses gymnastics in her fight with David, knows exactly where she is in the mall during the fight, comforts her sisters in an honest discussion of death and grief.
The animorphs change over the course of the series. They change into flies and tigers and bears and osprey. But they are also being changed by who they must be to survive this war.
