Walker is The Man. He's the guy that maintains the status quo in the Nightside for The Authorities. He does not live in the Nightside, he has a wife and two boys that live outside of the Nightside and don´t know anything about it.
He was also a friend of John Taylor´s father, and when he died, Walker secretly kept an eye out for young John.
As the Voice of the Authorities and "Overseer" of the Nightside, Walker has special priviliges and powers in order to do his job. The most famous and potent of his powers is his Voice. It is said that his Voice is derived from the original Voice, the one that said, "Let there be Light." and there was. It is a Voice that commands obedience and enslaves those who hear it. It is said that Walker once told a corpse to sit up and talk to him, and it did.
But although his voice is powerful, there are those immune to its effects. Namely John Taylor, though others have shown to be immune to it, if only partially.
Walker is said to be "protected in ways that are unknowable" and he had to be to enforce the rule of the Authorities in the Nightside, where monsters and gods roam freely and the Powers and Dominations of the supernatural world hang their hats.
He also has a portable Timeslip inside of a pocket watch that allows him to be when and where he needs to be.
He will do anything and sacrifice anyone in order to maintain the status quo.
Walker is not fond of the Nightside, in fact he hates it utterly.
Frequently he says that he'd like to "nuke the whole sick freak show and wipe it out of existence."