Gyatso was Aang's mentor and best friend.
Separation from Aang[]
Aang ran away from the Elder Council when they said they would separate Aang from Gyatso and cause him to move elsewhere due to him reaching the age of the knowledge of good and evil. Aang didn't want to live with anyone else and fled but eventually planned to return home. During a harsh storm Aang was forced under water and froze himself to prevent his death. The incident inadvertently saved Aang from the Air Nomad Genocide when an army of Fire Nation troopers led by Firelord Sozin wiped out their civilization and killed Gyatso.
Last Stand[]
Gyatso killed 100 Fire Nation soldiers before falling and personally faced off against Sozin before he fell.