Aang
Aang was a master airbender, proudly being given the blue tatoos before he had turned twelve. After running from his duties when the monks broke tradition, after Katara woke him up, he had focused, and decided to fight.
Aang was skilled in waterbending, approaching mastery, and now even Toph would admit he was a decent earthbender. And, Aang was even learning firebending…from Zuko of all people, the last person he had ever expected to count as a friend.
Despite his physical youth, Aang was 112 years old, a full century displaced from his own time, a time when peace had still held over most of the world. That was actually the hardest to reconcile with the world he'd returned to, that nothing was the same.
Aang had been forced to grow up a lot in the months since he had woken up from the iceberg, had been forced to give up a lot of his innocent beliefs. But, he was still only a twelve-year-old boy, and when the time had come, there was one thing his immature self could not let go.
So, despite how 'mean' it was, he couldn't help but feel pleased when they yelled at each other. In Aang's mind, that meant that Katara didn't like Zuko, and she could still be his, even though she had said nothing, had not acted differently, since their kiss on the Day of Black Sun.
And the rare times when he sees her not angry with Zuko, when she treats him almost normally, he feels jealous, like the spirits of fairness had gone on holiday and given what was his away.
But that was crazy, right?