Note: Characters who appear in the trilogy but are largely the same as their canon selves won't be listed here, as it would be mostly a rehash of their Avatar: The Last Airbender character sheet entries. Main series page is here.
Princess AzulaPrincess of the Fire Nation, last seen canonically in the midst of an epic Villainous Breakdown. After spending the next several months in a catatonic state, she was "rescued" by General Azun's men and set on a path to regain her sanity- and bring her into a confrontation with her most dangerous enemy...
General AzunA cousin of the Fire Nation royal family and a true fanatic to its cause, Azun genuinely believes that Fire Nation rule would be best for the world, and he'll go to almost any length to see it happen. His first goal- putting Azula, who he sees as the rightful heir, back on the throne.
Ling and JotiTwo peasant siblings Azula falls in with while amnesiac in "Heart of Fire".
Wei MingA mysterious girl with a strange power over darkness and a burning hatred for the Fire Nation- and Azula in particular. She was born a peasant in the Earth Kingdom and witnessed the destruction of her village and the death of her mother before becoming a Joo Dee in Ba Sing Se. Zhan Zheng found her, pulled her back from that state, and gave her a purpose- sharing her pain with the rest of the world.
Ty LeeA member of the Kyoshi Warriors, and Azula's ex-friend and sidekick from during the show. The two team up to track down Ursa and end up reconciling.
UrsaAzula and Zuko's mother, ex-wife of Fire Lord Ozai and a captive of the warlord Jian Chin.
Jian ChinThe Earth Kingdom warlord who captured Ursa. Ultimately allies with Zhan Zheng to become a godlike force of destruction.
IlookA psychotic waterbender with quicksilver mood swings and a terrible temper who hires himself out as a mercenary. He first meets Azula when working for a pirate captain but ultimately takes a job with Jian Chin. A spectacularly powerful waterbending warrior, his insanity is his greatest weakness, since it makes him terrible at planning or self-control. He's ultimately beaten in a waterbending duel by Katara and killed.
Zhan ZhengA powerful spirit, the embodiment of war, who was manipulating Azun, Wei Ming, and Jian Chin for his own purpose namely, reducing the world to a state of permanent chaos and conflict. It has a particular interest in Azula both because it feels she is the most like it of all the mortals it has encountered and because Zhan Zheng was "created" when Sozin committed genocide on the Air Nomads, and as a result all of its pawns manifest some sort of fixation on her.