"I wasn't there when the Fire Nation attacked my people. I'm gonna make a difference this time."
— Avatar Aang
Aang is a male Air Nomad born in 12 BG and the Avatar during the Hundred Year War. His immediate predecessor was Avatar Roku, and his immediate successor is Avatar Korra. As the Avatar of his time, he was the only person capable of using all four bending arts: airbending, waterbending, earthbending, and firebending. He was also one of a select few Avatars, and one of the first in many cycles to learn the ancient art of energybending, and the first Avatar known to have actively used the technique. Shortly before the beginning of the Hundred Year War, Aang was frozen in an iceberg for a hundred years; he later emerged, still biologically twelve years old, into a world engulfed by war.
Monk Gyatso was Avatar Aang's guardian, mentor, and father figure before the beginning of the Hundred Year War, as well as a member of the Council of Elders. According to Aang, he was the greatest airbender in the world during his time.
"We can't concern ourselves with what was. We must act on what is."
— Gyatso
"Selfless duty calls you to sacrifice your own spiritual needs and do whatever it takes to protect the world."
— Yangchen
Yangchen was the Air Nomad Avatar succeeding Avatar Szeto and preceding Avatar Kuruk. After mastering all the elements in her youth, she was eager to make a difference in the world despite her inexperience. She eventually became a respected figure and was hailed for her wisdom, determination, and power. Although Air Nomad culture emphasized pacifism and the sanctity of life, Yangchen was feared for her fierce willingness to do whatever was required to maintain balance and peace throughout the world in her duty as the Avatar.
Had Aang been present at the Southern Air Temple during the Fire Nation's assault on the Air Nomads, things may have gone a little differently for the airbenders. As shown during The Avatar Returns, Aang was already beginning to manifest the Avatar State at a young age, which may have been instrumental in fighting off the Fire Nation soldiers even during Sozin's Comet's flight through the sky. Realistically, however, it is more likely that Aang would have died from the sheer number of soldiers that assaulted his home and resulted in the next Avatar cycle beginning before Aang could become a fully realized Avatar.