Functional Order: Ne/Fi/Te/Si
Dominant Function: Ne
Aang is bursting with ideas, constantly thinking of new things to do, places to go, people to see. He’s good at attacking problems from many different angles until he finds a solution that works, and is a fast learner; he picks up waterbending so quickly from Katara that she begins to feel jealous. He is reluctant to acknowledge when he is out of options—learning earthbending (in which there is “no clever solution” and problems must simply be faced “head-on”) is especially hard for him, and it takes him a while to accept that killing Fire Lord Ozai is the only way to end the war. Even then, he ultimately manages to find another way to defeat Ozai
Auxiliary Function: Fi
Aang tends to keep his emotions to himself. He prefers not to burden others with his emotional baggage, trying to work things out on his own. He forces himself through days of mental and moral gymnastics before finally accepting that he must violate his principles and kill the Fire Lord to end the war. Even then, he is ultimately unable to go through with it, instead choosing to try to remove Ozai’s bending—a much risker proposition. When unlocking his chakras, he is unable to let go of his attachment to Katara, even though it means he will be locked out of the Avatar State and thus at a grave combat disadvantage.
Tertiary Function: Te
Aang sometimes has trouble deciding which ideas are practical and which are not. He has a strong work ethic and is determined to become a skilled Avatar. As an adult, Aang is instrumental in Republic City’s foundation and growth, realising his dream of a shared and diverse nation.
Inferior Function: Si
Aang is deeply affected by the loss of his culture and everyone he knew before the war. He’s upset to discover the changes made to the Northern Air Temple by its new inhabitants, and when Appa, one of the last remnants of his old life, is kidnapped, Aang becomes controlling and belligerent, behaving like an unhealthy ISTJ. As an adult, Aang favours Tenzin over his other children because he is an airbender and represents a chance for Aang to reintroduce the Air Nomads to the world
Note: This argument has been taken from Funky MBTI in Fiction.