I feel like ever since the ghost writers started writing the books the personalities went downhill. They all went extreme in their respective directions, and made it sort of unbelievable that they're still a team.
For instance, Rachel started out as a typical high school girl who's into looks and stuff, that just happened to be incredibly brave and tough in battles. I liked her in the beginning, but then she turned to this reckless blood thirsty lunatic. Yeah, I get it, it's war and the soldiers only get darker over time, but going extreme was bad for both the book and the character.
People may be able to justify Rachel going dark, but Cassie's growing conflict on moral issues is just dumb. It's war, and she should have figured out by then that the enemy's already been infiltrated, and the best she could do is defend the human race. I feel like the ghost writers had to go extreme with each character and Cassie's personality development really just makes no sense. Even for an environmentalist, you think about your own survival first, instead of worrying if the predator will starve if you don't offer yourself as dinner.
And then there's Jake, with practically no personality at all. He's the fearless leader but that's it. He makes the life and death decisions, he says the final word, but that's it. He has no personality, and even when the story's narrated by him, we don't really hear anything from him, just the fact that "as fearless leader he has to stay cool and be right". Why did Cassie and him like each other anyway?
Marco's a tad better, since Marco was originally just designed to be a comic relief person who's skeptical about everything. You can't really make an annoying person out of that even you went extreme.
Ax was okay I guess. Since he's literally an alien, everything weird about him could be forgiven. Even though he never learned after being on Earth for such a long time, it's still forgivable and at times he's funny. But still I wonder at times why didn't Ax acquire a lion for battles that would be seen by non-controllers...
I never quite got Tobias... He got issues, being an orphan, being a nothlit, belonging to 3 separate species... He could have been a nice person, but the fact that he's so deep in his personal **** really makes him annoying. And why did Rachel and him like each other anyway? The fact that Rachel's pretty probably explains his part, but what does Rachel see in this sad, introvert guy anyway?
So I guess at the end of the day only Marco and Ax are likable characters at all...