Ozai

Ozai also known as Fire Lord Ozai or the Phoenix King is a fictional character and the main antagonist in Nickelodeon's animated series  Avatar: The Last Airbender, despite the rarity of his appearance. He is voiced by Mark Hamill.

Season One
Ozai first appears in the episode "Winter Solstice Part 2: Avatar Roku", where in protagonist Aang's predecessor Roku reveals Ozai's plan to use Sozin's Comet to increase his firebending ability. In the episode "The Storm" he is shown in a flashback narrated by Iroh, where in Zuko speaks out against Ozai's actions, and for speaking out of turn, is challenged to an Agni Kai,or a duel by Ozai, upon refusing to fight, received a permanent scar, and is sent on a quest to find the Avatar, effectively exile. At the end of the season, Ozai sends his daughter Princess Azula to capture Zuko and Iroh.

Season Two
In the episode "Zuko Alone", it is revealed that Ozai spoke against the accession of his brother Iroh to the throne, on grounds of the latter's childlessness and withdrawal from the siege of Ba Sing Se. His father, Fire Lord Azulon was outraged at not only Ozai's questioning of his choice of successor, but also at his son's heartlessness to Iroh's loss. Fire Lord Azulon gave the order to kill his own grandson Zuko as pennance for his suggestion, whereupon his wife Ursa brought about the death of Azulon, and thus her own banishment, though the latter is not revealed until season 3. Ursa's actions concerning Azulon's death are not fully brought to light, but Ozai later considered banishment too light a punishment for what she had done.

Season Three
Ozai makes his first full appearance in the first episode of season 3, "The Awakening", after the Earth Kingdom capital of Ba Sing Se is conquered by Azula, wherein he welcomes the supposedly-redeemed Zuko and schemes to destroy the Earth Kingdom outright. When his enemies invade the Fire Nation during the eclipse, Ozai evacuates the capital city and hides underground, where Zuko confronts him. Ozai reveals Ursa's survival during their conversation, but when the eclipse ends and his Firebending returns, Ozai attempts to kill Zuko with lightning, but fails when Zuko redirects the attack, and his son later departs to assist the protagonists.

Sozin's Comet: The Final Battle
When told of the Earth Kingdom's continued resistance to his rule, Ozai decides to follow his grandfather Sozin's footsteps, and to use the power Sozin's Comet grants to Firebenders to carry out a genocide on the Earth Kingdom, as Sozin himself had done to the Air Nomads a century before. When the comet arrives, Ozai dubs himself the Phoenix King and declares himself ruler of the world, relinquishing his title of Fire Lord to Azula (though his new title of Phoenix King effectively makes Azula's title worthless). While attacking the Earth Kingdom, he is confronted by Avatar Aang, who after a lenghthy battle, becomes a fully realized Avatar, and ultimately strips him of his bending powers and thus of his right to rule. He is later shown imprisoned by Zuko, questioned on the whereabouts of Ursa.

Video game
He appears as an antagonist in the video game Avatar: The Last Airbender – Into the Inferno.

Film
Fire Lord Ozai appears in the live action feature film in The Last Airbender played by Cliff Curtis. Unlike the series, Ozai is a regular character in the film and shows compassion for his son, Zuko.