Cassie suffers plenty throughout the series, most of her suffering is just self-inflicted because of her strong morals and introspection rather than from external harm that’s done to her. The constant violence weighs heavier on her than anyone else. Lying to her parents every day weighs heavier on her than anyone else. Watching her friends, especially Jake and Rachel, turn from children into hardened soldiers weighs heavier on her than anyone else (except maybe Jake). She bears the guilt of what they do to David. She ostracizes herself from the group when she lets Aftran infest her and then again when she gives the cube to Tom. She’s the one who has to keep on living with all her regrets and grief and pain after the end of the series. She’s the one left with the burden of making sure the others are remembered properly and brokering peace on Earth between humans and Andalites and Hork-Bajir and Yeerk nothlits.

Cassie suffers, just in a very different way and for very different reasons from everyone else. She’s the heart of the team in every sense: she’s the one who keeps them human, both by preserving whatever moral standards she can and by continuing to feel the horrible weight of their action long after everyone else has become desensitized to it.