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Tear Jerker: Embers
Chapter 24, when Zuko tells Toph and Sokka to get Aang and to tell his uncle he wants him to look after him. He believes he's goning to die and has fully accepted it. Because its the best way to protect his people and to protect his sister. Who is probably going to deal the killing blow. When this troper actually thought about it in depth, I got teary-eyed. Also doubles as a crowning moment of awesome. How many people will go out and die like that?
"I know you'd try." Zuko smiled at her, faint as a ghost. "Tell Uncle... tell him I said to look after you, okay? For him." His face hardened. "Go!"
The scene in chapter 47 with Aang talking to Hakoda about the life he would have had: Peaceful, happy, raising a student and figuring out all kinds of new tricks with airbending...something wonderful and precious that he never really had a chance at, but he didn't know it, and he lost it forever.
In chapter 85, in a moment that mixes this and Awesome, Zuko finally engages in Calling the Old Man Out... to Iroh, and demands he declare whether he is loyal to his nephew, who he is the only friendly family member he has left, more or less, or to his precious Order of the White Lotus and the Avatar. Iroh's non-answer makes it clear enough that Iroh is quite blatantly driven away, with the obvious invocation that he and Zuko are no longer family to each other.