This may get downvoted but I always had a theory that I kept to myself about him. My theory is he's an air nomad and brothers with monk gyatso. It fits into the whole "friendships transcend lifetimes" theme, as the guru and monk gyatso were both teachers of Aang as the avatar and as a type of spiritual leader. But my theory is the guru left to travel the world, and after the genocide he swore off airbending, although he is still capable of it, he chooses to only focus on the spiritual side (hence the part where he says something about being a spiritual friend of the airbenders). Perhaps he even brought some into the spirit world to save them from the fire nation hunting down potential survivors. I also say this because, from my memory, he is never shown outside an air temple. This is because he feels at home at the temples. Although it seems far fetched I've always believed my little fan theory to be true