According to Lasciel, He Who Walks Behind is an Outsider, a "terrible creature, the most potent of the walkers, a powerful knight among their ruling entities." He is a hunter-spirit, a sort of supernatural hitman.
According to the ritual chant, he is the right hand and captain of The Lord of Slowest Terror, and a hunter of the shadows.
Dresden's encounter with He Who Walks Behind, had left an indelible mark on Dresden. A wizard using his Third Sight would be able to see it. Dresden beat long odds surviving him by using Fuego on a gas station.
In the seriesEdit
In Storm Front, a ThreeEye junkie tells Harry Dresden that he sees the things that follow Dresden, "I see the things that follow, those who walk before, and He who Walks Behind! They come, they come for you!".
In Blood Rites, Lord Raith and assistants had been using the ritual sacrifice entropy curse to kill throughout the book. At the end Lord Raith had Madge Shelly perform the final ritual curse that was intended to sacrifice Thomas Raith to kill Harry Dresden, using both of Margaret LeFay's sons for the purpose of ending the death curse Margaret left on Lord Raith. During the ritual Madge called up He Who Walks Behind, the guiding mind behind the curse. When He Who Walks Behind gains form as a spined cloud, He kills Madge, mangling her from the inside. He Who Walks Behind threatens Dresden and escapes the broken circle. (It doesn't explicitly say whether He Who Walks Behind was used only for this one use ritual curse or for all of the curses in the book. Most of them were a lot weaker with less at stake and Dresden is the enemy of He Who Walks Behind, not Arturo's employees)
In White Night, Lasciel also told Harry Dresden that He Who Walks Behind is an Outsider, the most potent of Walkers, and a powerful knight among the ruling entities—and Dresden overthrew him. She told Dresden that he was born under circumstance that would give him the potential to wield power over Outsiders.
In Ghost Story, Harry Dresden recalls the entire battle he waged with He Who Walks Behind in a convenience store after he ran from Justin DuMorne who wanted to enthrall him . He wounds Dresden in his spine and his head. It seems to delight He that Dresden can "perform under pressure" and Dresden lets "neither fear nor pain sway him from his objective"—as though He's testing Dresden. After it killed Stan, the store's cashier, Dresden becomes filled with rage and banishes He Who Walks Behind with his first use of "Fuego!" this was Dresden's first battle.
Later, talking with Bob inside his Skull, Dresden considers the possibility that He Who Walks Behind hadn't really tried to kill him, that instead, He manipulated Dresden into killing Justin. And, that He Who Walks Behind was quite possibly Justin's mentor as opposed to Justin being the one who simply summoned and sent Him after Dresden.
In Cold Days, Harry Dresden is no longer sure that He Who Walks Behind was actually trying to kill him that day when he was sixteen and fought him. Dresden now thinks that He was "shaping" him though Dresden doesn't know what for.