Meeting John Thain in person is a bewildering experience for anyone who has had much contact with C.E.O.'s or Wall Street moguls. His manner is that of a distant relative at a family reunion, friendly in an unforced way. He speaks slowly in a high-pitched Midwestern accent. He is anything but slick—"a quieter personality," notes Robert Rubin, the former Treasury secretary who nurtured the young Thain in his early years at Goldman Sachs.

And yet here is John Alexander Thain, suburban dad and former high-school wrestler, installed at the top of Merrill Lynch during one of the most tumultuous moments in finance. It is white-knuckle time at Merrill. In addition to its subprime woes, which have infected all of Wall Street, this old-school, tradition-conscious firm is suffering from a kind of corporate post-traumatic stress disorder following the divisive, nearly disastrous tenure of Thain's predecessor, Stan O'Neal. Not since the Salomon Brothers scandal, in 1991, when Warren Buffett arrived on a white charger to confront a Treasury-bond-trading debacle, has a new C.E.O. faced such a variegated mess. Even compared with other questionable deals in the current crisis, the subprime-mortgage ventures at Merrill were freakishly ill-advised. Some $24 billion has had to be written off of the company's balance sheet, making Merrill one of the names, along with Lehman Brothers and Morgan Stanley, on the whisper list of Wall Street firms that could join Bear Stearns in an epic meltdown. Thain has the unenviable task of preventing that from happening—and of preserving Merrill's independence amid a plunge in the company's stock price, a flurry of subprime-related lawsuits, and any number of state and federal investigations. (View slideshow of some of John Thain's rivals and former colleagues.)

As one of the earliest traders in mortgage-backed securities—with a résumé that includes engineering the rescue of the mega-hedge fund Long-Term Capital Management in 1998, the collapse of which seems eerily familiar today—Thain is no stranger to either the intricacies of mortgage bonds or the delicate job of extricating firms from self-created trading fiascoes. He overhauled the decrepit, conflict-plagued New York Stock Exchange after stepping in to replace longtime C.E.O. Dick Grasso in 2003. Thain is, in short, less a technocrat than a steely corporate warrior, having emerged victorious from both a Goldman power struggle and the poisonous infighting of the N.Y.S.E. He brings to the investment bank—and to America's rattled financial markets—a steadiness to match his Buzz Lightyear looks. This may be precisely what banks like Merrill Lynch need just now: the nonflashy executive who, like Goldman's Lloyd Blankfein, is successful precisely because he is so nondescript.