I mean out of the leads, Katara, Zuko, Aang, Sokka, and Toph? I think everyone's pretty much in agreement about Zuko being the most complex out of the whole show (and definitely out of those five leads), but I'm noticing now (after rewatching the previous eps) that all the characters except Aang had some sort of major change to their character that lasted until the present episode. Zuko had/has his inner turmoil, Toph had the "I'm a loner" thing that eventually went away, Sokka had/has his lost love(s?) and the sexist thing, and Katara has matured and dealt with her jealousy of Aang's skills (oddly enough by becoming?? a lot better then him)...
But Aang, as I rewatched, seemed more like a person we've just met rather then a developing character. I mean the first time you meet someone they present a certain image that gets stuck in your mind (in this case, that he was so relentlessly happy). Then the first time you see them truly angry, it shocks you, but it's not development, it's just a side of the person that you simply hadn't seen before. That's what every change to Aang seems like to me, like it's just something that was always there to begin with but we as the audience just hadn't seen yet (versus, let's say Zuko who we've seen without patience and then slowly saw him mature and have patience). Agree or disagree???