Order of the White Lotus:
The Order of the White Lotus is basically a group of old warriors spanning across the four nations who seek everlasting peace in the world. Though they don't appear until season 3. The most prominent members are Jeong Jeong, Iroh, Bumi, Pakku, and Piandao. All of them are masters of their specific art and Aang and his friends met each of them on their journey. Since some of them don't have much backstory, I'll list a bit about them and their introductions in the series.
|left to right: Jeong Jeong, Bumi, Iroh (center), Pakku, and Piandao|
Bumi: The mad king of Omashu (I've already talked about him so I won't repeat myself).
Uncle Iroh: The Dragon of the West (again I've already talked about him).
Pakku: The greatest waterbending master in the Northern Water Tribe.
Piandao: A great swordsman and the one who taught Sokka the way of the sword.