Fire Lord Zuko

Crown Prince Zuko was the enemy in the show.

As a child, his little sister Azula was their father's favorite. Granted, he was their mother's favorite, but she disappeared. He grew up hearing that Azula was lucky, and he was lucky to have been born. When a war meeting came up, he decided to join. But when he spoke out against a general, he was challenged to an Agni kai! He was not afraid to fight, thinking it was the general he'd be fighting. But since he was in his father's chambers, it was he who he had dishonored, and he who he would have to fight. Zuko begged to be let off, but Ozai refused and scarred him for life. He then banished Zuko( who along with his uncle was considered a traitor)  until he had captured the Avatar, who some thought was still alive. His Uncle Iroh stayed with him during his exile, and was more of a father to him then Ozai.

Zuko of course discovered that he was alive and spent awhile fighting him. But when he joined his sister and she felled Aang, she told Ozai that Zuko had done it. Hence Zuko was welcomed back. In the episode, The Avatar And The Fire Lord, he discovered how the war began, and that Aang is his great-grandfather. Well, Roku is, anyway.

When Aang invaded the Fire Nation during the day when all lose firebending, Zuko stood up to his father, saying the things that his uncle had tried to get him to realize. He then said he would help the Avatar defeat Ozai, that was his destiny. Ozai tried to kill him right then, but Zuko redirected his lightning, and disappeared.

He joined the Avatar, but his firebending had diminished now that his anger had as well. He regained it and taught Aang firebending. He than defeated Azula at her coronation, and became fire lord. He married Mai and had a daughter