Zuko, born in 83 AG, is the firstborn son of Fire Lord Ozai and Princess Ursa. As a child, he was empathetic but struggled with firebending, causing Ozai to look down on him. Zuko and his sister Azula grew apart as she mocked and manipulated him, while their mother favored him. After their cousin Lu Ten died, Iroh abandoned the siege of Ba Sing Se, Ozai became Fire Lord, and Ursa mysteriously disappeared. At thirteen, Zuko opposed a general’s plan to sacrifice soldiers. Ozai considered this an insult and forced Zuko into an Agni Kai. Refusing to fight his father, Zuko begged for forgiveness, but Ozai scarred his face and banished him. He was told he could only return by capturing the Avatar. With Iroh beside him, Zuko spent years searching for Aang. He repeatedly battled Team Avatar, clashed with Commander Zhao, rescued Iroh, and secretly operated as the Blue Spirit. During the Siege of the North, he captured Aang but was forced to flee after Zhao’s defeat. Branded a traitor, Zuko and Iroh became fugitives. After separating from Iroh, Zuko lived under a false identity, eventually reuniting with his uncle in Ba Sing Se. In Ba Sing Se, Azula convinced Zuko to betray Iroh and join her. He returned home as a hero, but discovered that Azula had lied about killing Aang and that Ozai planned to destroy the Earth Kingdom using Sozin’s Comet. Horrified by his family’s actions, Zuko confronted Ozai during the eclipse and chose to leave the Fire Nation. Zuko sought out Aang’s group, but they initially rejected and distrusted him, especially Katara. He eventually proved his sincerity by protecting Aang from Combustion Man and defeating the assassin he had hired. Having finally earned their trust, Zuko joined the group and became Aang’s firebending teacher.

Aang is a 12-year-old Air Nomad who discovers he is the Avatar, the person responsible for maintaining balance between the four nations. Frightened by his new responsibilities, he runs away and becomes trapped in an iceberg for 100 years. Katara and Sokka discover and free him, revealing that the Fire Nation has destroyed the Air Nomads and started a war. Aang decides to end the war and travels with Katara and Sokka to master water, earth, and firebending. During his journey, he faces many challenges, learns about the Avatar State, and gains allies, including Toph, a powerful earthbender who becomes his teacher. Aang repeatedly encounters Prince Zuko, who hunts him to restore his honor. Zuko eventually rejects the Fire Nation’s path and asks to join Aang’s group. Although the others distrust him, Aang accepts Zuko as his firebending teacher.

Toph Beifong, born 88 AG blind into the wealthy Beifong family, was sheltered and hidden from society by overprotective parents who saw her as fragile. At five, she ran away and learned earthbending from blind badgermoles, developing seismic sense to "see" through vibrations. Secretly, she became a champion in underground tournaments as "the Blind Bandit." At twelve, she met Aang and joined Team Avatar as his earthbending teacher, escaping her restrictive parents. She clashed with Katara but stayed, taught Aang through confrontation, and discovered metalbending while imprisoned in a cage, becoming the first person to bend metal. She helped thwart Fire Nation plots in Ba Sing Se, escaped the city's fall with a wounded Aang, and later fought Fire Nation soldiers during the Day of Black Sun invasion. At the Western Air Temple, Toph was the only one to immediately sense Zuko's sincerity when he arrived seeking to join them, and the group ultimately welcomed him after he helped defeat Combustion Man.

Katara and her brother Sokka discover Aang, the last Air Nomad and Avatar, trapped in an iceberg. They free him and join his quest to master the four elements and defeat the Fire Nation. Katara becomes Aang’s waterbending teacher and one of his closest friends. Along their journey, they face Fire Nation enemies, help people affected by the war, and visit the Northern Water Tribe, where Katara becomes a powerful waterbender. In the Earth Kingdom, they meet Toph, who joins the group as Aang’s earthbending teacher. Meanwhile, Zuko continues hunting Aang but begins questioning his loyalty to the Fire Nation. After betraying his uncle Iroh, Zuko eventually rejects his father’s path and asks to join Aang’s group. Though Katara distrusts him, Aang accepts Zuko as his firebending teacher.

Sokka is the son of Chief Hakoda and Katara’s older brother. After their father leaves to fight in the war, Sokka helps protect their Southern Water Tribe village. He and Katara discover Aang trapped in an iceberg and join him on his mission to defeat the Fire Nation. Although Sokka cannot bend, he becomes a valuable member of the group through his intelligence, humor, planning, and warrior skills. During their journey, Sokka helps the group survive numerous dangers and gradually becomes a capable leader. In the Northern Water Tribe, he falls in love with Princess Yue, who sacrifices herself to restore the Moon Spirit. Sokka later travels with Aang and Katara through the Earth Kingdom, helping Aang find an earthbending teacher and welcoming Toph into the group. He repeatedly faces Zuko, who initially hunts them but eventually rejects the Fire Nation and joins the group as Aang’s firebending teacher.