So I have two possible ideas for Aang and I'll let you pick if you like either. First scenario, he could be Belle's younger brother and maybe help her work on the farm. They wouldn't have any parents as of yet or any other relations.

Or he can be a kid Belle takes off the streets and allows to help her work on the farm. Maybe she can help him seek out his mother should he get one.

i'm in the wrong journal for this but oh well