Sister Iio was an Air Nomad nun, as well as the Mother Superior of the Eastern Air Temple during Aang's childhood. She always knew best, but she was so unaffected and cheerful that no one really minded.
In 6 BSC, Iio guided a group of young Air Nomad children through the tradition of choosing a flying bison. She met the group of youngsters, among which was Aang, and led them to an open area. She gave each of them an apple to lure a bison calf, and advised them to choose carefully when picking out their bison, as "a sky bison [was] a companion for life."