Fire Lord Ozai
The second son of Fire Lord Azulon, Ozai is the current Fire Lord. He has two children, Zuko and Azula, is the brother of Iroh, and husband to Ursa. Ozai is a master Firebender, a ruthless man, and hungers for more power. He strongly favors Azula over Zuko, who greatly resembles him and his ideals, along with being a firebending prodigy.
Ozai banished Zuko from the Fire Nation and revoked his birthright when he was 14, for being "disrespectful" in a war room meeting and "showing shameful weakness" when he didn't fight Ozai during an agni kai. Ozai gave Zuko his facial scar at the end of said agni kai, and the only way Zuko could return home was if he captured the Avatar. Who, at that time, hadn't been seen in almost a century. Yeah, good luck with that. -_-;
Ozai originally was not heir to the Fire Nation throne because Iroh was older. But when Iroh's son Lu Ten died and Iroh fell apart, Ozai saw an opportunity and grabbed it. He requested an audience with his father, where after showing off Azula's talent in firebending and knowledge, humbly asked Azulon to revoke Iroh's birthright and use him instead. However Azulon favored Iroh more, and was outraged at what Ozai was suggesting, especially right after Iroh's loss. Azula had been secretly watching their meeting, and if she was telling the truth, Ozai's punishment was to understand the loss of losing a son by sacrificing his own. Ursa caught wind of this and being very protective of Zuko, did something to change it. The next morning Ursa could not be found, and Azulon had died. As was his "dying wish", Ozai was crowned Fire Lord.
Ozai has been seen in flashbacks and mentioned many times, but he has yet to give a present-day appearance in the show (that doesn't including being a shadow in the background). In Book 1: Water, Ozai's features had been shadowed, but in Book 2: Earth, Ozai was shown in normal lighting, albeit in a flashback. He... seems to look normal, though his eyes are never shown up close. He wears the Fire Nation crown, which is a golden flame-shaped piece in his topknot, and has long hair and a chin-beard.