She's also the one shown doing the most nice things for people, is the most academically successful, and most functional in the real world. Cassie's morality and kindness are almost exclusively of the "tell, don't show" variety. Everyone talks about how good and nice she is, but it mostly manifests in indulging her own interests in animals. She helps animals for the same reason I applaud football players - because that's what we're interested in as a matter of taste. Meanwhile Rachel jumps into crocodile pits to save children, when the common perceptions of her articulated by her supposed friends would have her either shrugging off the kid's death as Darwinian justice, or else jumping into the pit to engage the crocs in bloody combat to the death, regardless of the child's safety.

The other Animorphs spout off that "born to be a viking" garbage to make themselves feel better about being such relative pussies next to a girl who is pretty much better than them at everything besides veterinary medicine and video games, and to alleviate their guilt for letting her do all their dirty work, when her own narrative stream of consciousness shows she is as bothered by it as everyone else. By contrast Marco almost never shows remorse for anything, aside from possibly killing his mother, in highly justifiable and necessary circumstances (and in those situations, it's generally Rachel comforting him and offering him the affirmations no one ever gives her), and also has a strong streak of cowardice and overtly selfish behavior he tries to lampshade by joking about it.

This is a girl who's so dumped on that she dates a guy/bird who has very little interest in actual dating or spending more time with her than he absolutely has to, except on his terms, pretty much because he's the only person who appreciates her moral character and never once expresses worry about her violent tendencies.