there's a lot of interesting discussion on TVTropes, since the Animorphs tend to follow the formula of a "
Five Man Band," which works this way:
The HeroThe Lancer (2nd in command, has tension with the Leader but also may be their best friend. Think Lancelot to Arthur.)
The Big Guy (The muscle, could be dumb, could be smart, but he's the one you always go to when you're in a physical pinch)
The Smart Guy (The nerd with the computer and glasses who always talks in tech-speak)
The Chick (I think they may have changed this to "The Heart" to be more PC, but essentially, they're the person who keeps everyone in line moralistically and offers moral support.)
All those articles go into it a lot more deeply.
I mean, it's hard with the Animorphs because they share a lot of the characteristics. It's clear that Jake is the hero, and Rachel is mostly The Big Guy (but sort of the Chick b/c she likes *shopping* and *girl stuff*), but Marco, Cassie, Tobias, and Ax are a little harder to categorize. Ax is mostly the smart guy with his Hitchhiker's Guide level knowledge of the galaxy, but so is Cassie, because she always tells everyone what to expect when they use a new morph. Cassie is also sort of the chick, because she *cares about the animals* and *always knows when something is bothering someone*, but so is Tobias, because he's very in touch with his emotions (lol) and he was sort of an outcast because he was a nothlit. BUT Tobias also has aspects of the Lancer, since he does lead the trio of him, Marco, and Ax, and he often contradicts Jake on the important stuff. Then there's Marco, who I personally think is the Lancer, especially since he's Jake's bff, and because he was most opposed to being an Animorph at first, but even he has aspects of The Big Guy (gorilla morph) and The Smart Guy (go-to strategist.)
Anyway, the point is, I like that they're hard to pin down, it shows the depth of their characters that they all had opposing strengths and weaknesses and they aren't just clear-cut archetypes. And, to be totally fair, I think they all played aspects of "second in command" at different points in the series, and they mostly decided everything together anyway, so there was never any real hierarchical structure, even with Jake being the leader.