Who is Keyser Soze?

From The Usual Suspects:

Verbal (Kevin Spacey): Who was Keyser Soze?

Verbal (Kevin Spacey): He's supposed to be Turkish. Some say his father was German. Nobody ever believed he was real. Nobody ever knew him or saw anybody that ever worked directly for him, but to hear Kobayashi tell it, anybody could have worked for Soze. You never knew. That was his power. The greatest trick the devil ever pulled was convincing the world he didn't exist. [...] and like that, poof, he's gone. Underground. Nobody's ever seen him since. He becomes a myth, a spook story that criminals tell their kids at night: "Rat on your pop and Keyser Soze will get you." And no one ever really believes.

Dave Kujan (Chazz Palminteri): Do you believe in him Verbal?

Verbal (Kevin Spacey): Keaton always said: "I don't believe in God, but I'm afraid of Him." Well I believe in God, and the only thing that scares me is Keyser Soze.