Ken Griffin isn’t a people person. The billionaire founder of hedge fund Citadel has proved himself an adept reader of markets and a savvy investor, thriving in areas of finance where an otherworldly mastery of electronic markets takes precedence over personal relationships. Back on earth, Griffin is notoriously hard to work for. Case in point: Kevin Turner, the former chief operating officer of Microsoft, who lasted only seven months in the hothouse of Citadel’s Chicago headquarters before unexpectedly leaving on Jan. 27. Griffin hired Turner in July as the first chief executive officer of Citadel Securities, a market-making business that acts as a middleman for its customers, trading stocks, options, swaps, and Treasury bonds. Griffin’s objective is to transform the little-known operation into a powerhouse that facilitates buying and selling in an array of securities for investors around the world.