Aang
After sleeping in the Avatar State in an iceberg for 100 years, Aang wakes up to find out he is the last surviving airbender following the fire nation's attack. As the Avatar, the only one capable of mastering all four elements, it is his destiny to bring peace to the world. Thankfully, siblings Katara and Sokka join him on his journey to master the other three elements and the Avatar State itself.
Katara
As the only remaining waterbender in the Southern Water Tribe, Katara joins Aang on his journey to the North Pole, in the hopes that both of them can learn more waterbending from a master. Although she and Sokka bicker a lot, they've grown to depend one another. Katara's abilities increase dramatically throughout the show, and she becomes Aang's main waterbending teacher and an accomplished healer.
Sokka
Even though Sokka is a non-bender, he still recognizes Aang and Katara need support, and he joins them in their adventures. Over the course of the series, he becomes not only a trusted ally but a brilliant strategist, inventer, and master swordsman. Sokka also carries his beloved boomerang, which always come back to him.
Toph
She may be blind, but Toph is a formidable earthbending master, having learned to earthbend through seismic sense from badgermoles, the original earthbenders. Team Avatar discovers her in an earthbending battle ring, and she joins them as Aang's teacher. Later in the series, she becomes the first person ever to discover metalbending.
Zuko
Zuko is the exiled prince of the Fire Nation, tasked with capturing the Avatar in order to restore his honor. Although he desperately wants to regain favor with his father Ozai, his uncle Iroh gently teaches him to find a new path, and he eventually joins Team Avatar as Aang's firebending teacher.
Appa
Appa is an air bison, capable of sustained flight through airbending. Having been Aang's closest companion before they were both frozen in the iceberg, Appa remains loyal to Aang and his friends, and becomes their primary mode of transportation across the globe via a saddle on his back.
Momo
Momo is Aang's other animal companion, a flying lemur who joins the group when they visit the remains of the Southern Air Temple where Aang grew up. Momo is often a source of comic relief and is usually food-motivated.
Iroh
After losing his son in an attack on Ba Sing Se, Iroh retired from being a war general and sought peace in nature and his family. When Zuko is scarred and exiled by his father, Iroh goes with Zuko in an attempt to teach him both firebending and more pacifist ideals. He famously loves tea.
Suki
Suki is the leader of the Kyoshi Warriors, a group of female fighters who defend their island named after Kyoshi, the Avatar two incarnations before Aang. Like Kyoshi, they fight with metal fans. Suki becomes an important friend and ally to Team Avatar, and Sokka's girlfriend.
Roku
As Aang's previous incarnation, Roku sometimes appears to the young Avatar in spirit form to offer guidance. Roku's murder by his former friend Firelord Sozin brought about the start of the 100-year war.
Azula
Azula is Zuko's younger sister, a firebending and lightningbending prodigy, and one of Team Avatar's greatest foes. With Zuko in exile, she becomes her father's greatest weapon, but without a stable support network, Azula slowly crumbles under the pressure and loses her sanity.
Mai
Mai is initially an enemy of Team Avatar as an expert knife thrower and one of Azula's companions. Eventually, she betrays Azula out of love for Zuko and is subsequently imprisoned.
Ty Lee
Ty Lee is an expert martial artist who uses chi blocking to fight against Team Avatar as Azula's companion. After Mai betrays Azula, Ty Lee also betrays her for Mai's sake and is imprisoned. Eventually, she joins the Kyoshi Warriors.
Ozai
As the current Firelord, Ozai intends to use Sozin's Comet, which grants firebenders a huge power boost, to finally conquer the Earth Kingdom and rule over the entire world.