Since graduating from Princeton 25 years ago, Feinberg has never given an interview and has never been photographed by the press. Not that there has been much demand until now. On Wall Street, the C.E.O. of Cerberus Capital Management, an investment firm with $26 billion in assets under management, has long been admired. (“You probably think you’re smart,” says one former employee. “Now take your brain and mine, take them to the 28th power, and you have Steve Feinberg.”)