Who is Malachi Constant from The Sirens of Titan and what is their importance?

Malachi Constant is the main character throughout the novel, taking on at least four different identities over the course of the story.

Constant is the wealthiest man in the world, owing to the fantastic success of the company he has inherited from his father, called Magnum Opus. He is a playboy, partying his way through life in the wake of his seemingly incredible luck in making money. As his financial fortune crumbles, however, Constant finds that he has nowhere to turn.