Well we can guess the views (from interviews and subtextual reading of her books) of KA Applegates and they are almost the essence of political correctness. (an ideology that is generally anti war if the west does it, morally libertine, mildly socialist in economics i think someone else discussed the themes of animorphs) KA is very anti-war which is why Cassie and Jake's relationship falls apart...
Rachel, despite being KA own creation, was still offensive to the authors world view: she was violent, materialistic, vain, rude... to my own world view i would describe rachel as: brave, stylish, proud, and honest... Rachel, like Jake and Cassie's relationship, died of theme. Cassie, who epomitises politically correct values was the only one allowed to survive the war.
Ironically I could be quite wrong... long before i thought that Rachel died of theme, I had a strong impression that Naomi was a social conscience left leaning/PC lawyer (this is California after all so those values are quite likely for her)...