In person, Griffin is indistinguishable from a midlevel banker, analyst class of 1996. "He’s not going to put on a show for you—‘Let me be really nice to the press, but really I’m a bullshit billionaire,’ " says Griffin’s friend Ian Bremmer, the president of Eurasia Group, a political risk consultancy. "If you just met him in a bar, you would not guess he’s a billionaire."

Of medium height and build, Griffin is dressed in a slightly rumpled navy suit and has great, bushy eyebrows and a fringe of graying hair spread across his forehead. His eyes, which are dark blue and set close together, lock onto you when he delivers one of his carefully honed and spun messages, which he tends to issue vigorously and often. A favorite topic is the boundless opportunities for businesspeople in America.

Ken Griffin showed an early aptitude for two things that have led many a math nerd to strike gold in the modern economy: computers and business. His father’s career with General Electric took the family around the country, but they eventually settled in Boca Raton, Florida, where his dad started a building supply company and his mother raised Griffin and his younger brother and sister. Bright and entrepreneurial, Griffin dabbled in computer programming as a student at Boca Raton Community High School, eventually shipping off to Harvard. (A former dorm mate, Lisa Borodkin, recalls that he "kind of kept to himself" while there.) After reading an article in Forbes, Griffin became intrigued by the stock market and began trading. Soon, he cobbled together a small investment fund.

Following his graduation in 1989, he bounced around Florida, trying to scare up investors for his nascent hedge fund. A seasoned moneyman named Frank Meyer met Griffin through mutual friends and facilitated his Lana Turner moment: Meyer offered to back Griffin if he moved to Chicago, where Meyer’s company was based. Griffin agreed. That arrangement formed the basis of Citadel, in which Meyer remains an investor.

Every winning hedge fund manager has a special sauce. SAC’s Cohen became known for making rapid-fire trades, using an information edge to get in and out of stocks. D.E. Shaw’s David Shaw is obsessed with science, devising proprietary computer models to help him trade.

Griffin melds two approaches: He’s a numbers wonk, focused on spreadsheets and technology, but he is also fixated on management theory and has worked to build Citadel into an agile player that can pounce when other funds run into problems. Though he’s not a phone thrower, Griffin can be unforgiving on the trading floor, and notable turnover at his company may attest to his reputation as a difficult boss; more than a dozen senior managers have reportedly left Citadel in the past few years.

Yet for all his prickliness, Griffin has produced. Citadel has had average annual returns of more than 20 percent since 1998. In 2006, the fund’s return was 30 percent—believed to be the second-best hedge fund performance, after SAC’s.

A hedge fund, of course, is nothing more than a lightly regulated pile of money, albeit one that has created more rich people in a shorter period of time than any other innovation in modern finance. And hedge fund managers pay themselves generously: typically a management fee of 2 percent of assets and a performance fee of 20 percent of profits at the end of the year.

Given the windfall potential of such an arrangement, it’s no surprise that more than 9,000 funds have sprouted up in the past 12 years. The value of the assets these funds are managing has doubled since 2002, to over $1.2 trillion, and it hasn’t made life easier for any of them. "After [collapsed hedge fund] Long-Term Capital [Management] blew up, there wasn’t low-hanging fruit, there was fruit rotting on the ground," Griffin says. Now "all the rotting fruit on the ground is long gone, all the fruit on the low hanging branches is long gone. And now you’ve got to go find a ladder and pick the fruit off the very top of the trees."

Citadel’s headquarters are housed in a downtown Chicago skyscraper; the somber lighting and architectural floral arrangements seem as if they could be in the lobby of an Ian Schrager hotel. Even though more than 1,000 people work at the company, during a visit, a hermetic silence permeates the place, as rows of young men in khakis sit staring at flat-screen monitors.

The fund invests around the world using a variety of strategies. Griffin likes a personal touch: Throughout the 1990s, the fund posted impressive returns by following Griffin’s gut and investing in esoteric areas, from Japanese warrants to convertible bonds. In 2001, the company charged into energy, hiring a handful of Enron traders after that company filed for bankruptcy. Indeed, Griffin has become known for finding opportunities when trouble hits markets, as when hedge fund Amaranth Advisors blew up this past fall. Citadel and Amaranth had a close relationship—Charles Winkler, Amaranth’s chief operating officer, had previously held the same position at Citadel—and when the distressed fund was forced to sell its energy holdings in a panic, Citadel (along with J.P. Morgan Chase) stepped in and bought them. In a March letter to investors, Citadel disclosed that its Amaranth-related trades accounted for several hundred million dollars of its soaring 2006 performance.