We'll never really have a solid answer to that question. My first impression upon watching the film as a kid was the obvious "Verbal Kint was Keyser Soze"; of course, when I got older my way of thinking changed a bit, and I got around to thinking that Keyser Soze was basically just a criminal organization that crafted the legend of the man in order to create an air of mystery around their activity. I don't think that a crime lord such as Soze - if he existed as a single man - would really ever come that close to getting caught high and dry, and for that reason I don't really think he could be just one man. He'd have to be an entire corporation, a la Murder Inc.