Who is Marcus Beaumont from Ninth Key and what is their importance?

Marcus Beaumont is Thaddeus Beaumont's brother, and the novel's main antagonist. He is described as an evil man, who lets nothing stand in his way when it comes to developing land and building and maintaining a booming business. When Susannah discovers that Marcus murdered the missing environmentalists, she lets Jesse take care of justice. Marcus will experience bedevilment by the spirits of the people he killed until he eventually dies himself.