Cohen, 47, is the most powerful trader on Wall Street you've never heard of.

Cohen, 47, is the most powerful trader on Wall Street you've never heard of. The founder of SAC Capital Advisors, a highly secretive and stupendously successful $4 billion group of hedge funds that bears his initials, is considered to be a market genius by even his harshest critics. His firm routinely accounts for as much as 3% of the New York Stock Exchange's average daily trading, plus up to 1% of the NASDAQ's -- a total of at least 20 million shares a day. And while most of his rivals struggle to keep their trading costs down, "Stevie," as he's known on Wall Street, is one of the few to pay full freight. He hands over about $150 million a year in commissions to Wall Street, making him one of its 10-largest customers.