You know that trend where it's like "guy who would burn you for the world" vs "guy who would burn the world for you"? If you just heard of ATLA and looked at Aang and Zuko's character you'd think that the first would be about saving the world first and the second about burning the world.... But nah. It's the opposite.
Aang was ready to let the world be at complete war if it meant he could keep on loving Katara. Meanwhile Zuko quite literally broke up with Mai to save the world.
Heck, you can even look at it from how they both deal with Ozai. Aang has doubts about killing him because of his attachment to his roots and what remains of it. Meanwhile Zuko straight up go to his father after everything is done and dealt with just to tell him that if he had gotten the chance he would have killed him himself.
Aang is very much a "burn the world for you" guy while Zuko is very much a "burn you for the world" guy.
Which is why they also complete each other so well. Zuko needs someone that will keep him attached to the people he cares about when his own morals are dead set on the good of the world. Meanwhile Aang needs someone to remind him that there are bigger things at stakes than just his close friends' lives. Aang is the savior of the world and inherently a selfish person. Zuko is the runt of Fire Nation and inherently a selfless person.

















