Iroh is
Zuko's uncle. He had lost his son in his past life and has fear of the spirits. He turned on the Fire Nation, since
Ozai tried to kill the moon spirit, and now he is a traitor who is on the run.
Iroh is really the only person who keeps Zuko from totally losing it. Iroh has always been looking out for Zuko, even before he was banished. Though Zuko may not always seem respectful of his uncle's support, you know that deep down he does appreciate it, and in the later episodes
you can see signs of that. Iroh's wise and laid-back nature balances out Zuko's impatience and rage, making them a great duo and leading to many comedic scenes.