I ran across this thread off a google search, and i saw the question of what an outsider is. Im a big fan of Harry. In actul myth if your interested an outsider is a demon/entity who is outside the jurisdiction of either Good, or Evil. They are mentioned in the bible actuly. They are refered to as entities which fall outside the control of either Heaven, Hell, or the forces which maintain the balance between the two.

Outsiders also take places of power in some older religions, Cthulu myths are an example. As well as Aztec, Myan, Navajo, even Greek and Roman.

He Who Walks Behind is merely one, and one of the more well known ones.