The Voyage of the Dawn Treader
Eustace and Edmund
Eustace, the unpleasant jerk of this book, is often contrasted with Edmund, who was the unpleasant jerk in the first Narnia book. At one point, Edmund himself makes the comparison explicit, telling Eustace, "Between ourselves, you haven't been as bad as I was on my first trip to Narnia. You were only an ass, but I was a traitor" (7.54). As Edmund's comparison suggests, Eustace may be more unpleasant to be around, but at least he's not actively working for the side of evil. Edmund and Eustace are both redeemed by Aslan, but for Edmund, Aslan must actually sacrifice himself; for Eustace, Aslan just helps him peel off the symbolic shell he has made for himself (made literal in the book by his peeling off his dragon skin).
Eustace and Caspian
Eustace, who starts out weak and cowardly, thinks of himself in contrast to brave, strong, kingly Caspian. When he is turned back into a boy after his stint as a dragon, Eustace tells Edmund that "You'd think me simply phoney if I told you how I felt about my own arms. I know they've no muscle and are pretty mouldy compared with Caspian's but I was so glad to see them" (7.44). Eustace's experience as a dragon has made him accept who he is and value what strength he does have rather than constantly comparing himself to Caspian, as he used to.