Introversion doesn't necessarily correlate to 'talkativeness in a group.' I'm an introvert, but if you put me in a social setting, I will make pointless fun conversation with everyone else all the damn time, too. I just eventually need a break from that, I can't do 'social' in the long-term.
I've heard it said that introverts 'gain energy' from being alone. They can be social, but they need to be alone to 'recharge.' Extroverts, though, 'gain energy' from being around others. They, similarly can be alone, but for them it's being alone that drains them, and they need other people to get that energy back.
Based on this definition, yes, I definitely would call Tobias an introvert. He contributes to group meetings and interacts with the other Animorphs whenever it's relevant, sure. But he seeks out alone time when he needs energy, the way introverts do.
Ax is probably an introvert, too (although, like XF said, as an alien we can't really apply human psychology to him). I'd say Jake and Cassie are 'middle-verts,' not really one or the other (Jake might be slightly extroverted, Cassie slightly introverted), and Rachel and Marco are definitely extroverts.
Ooh, somebody needs to figure out the Animorphs' Myers-Briggs types! Or, if not, I'll probably end up doing it (but after work when I have time).
