He , along with his younger sister Katara , discovers an Airbender named Aang , the long-lost Avatar , and accompanies him on his mission to win the Fire Nation and bring peace to the world . He , along with his younger sister Katara , discover an Airbender named Aang , the long-lost Avatar , and accompanies him on his quest to defeat the imperialistic Fire Nation and bring peace to the war-torn nations .