Ozai
Ozai first appears in the episode "Winter Solstice Part 2: Avatar Roku" in a vision where Avatar Roku explains to Aang that Ozai plans to use Sozin's Comet to increase his firebending ability. In the episode "The Storm" he is shown in a flashback of a story being told by Iroh explaining why Zuko is obsessed with capturing the Avatar. Zuko is challenged to a duel by Ozai and when Zuko refuses to fight, Ozai burns his son's face and banishes him. Zuko now tries to capture the Avatar to restore his honor and please his father.  At the conclusion of the season 1 finale, "The Siege of the North, Part 2", Ozai sees Zuko as a failure and believes Iroh is now a traitor to the Fire Nation. Just before the episode ends, he is seen giving a task to Azula, though the details of this task are not revealed until season 2.