Callan says that she was able to bond with hedge fund managers and bring lucrative new business to Lehman at least partly because of her gender and her ability to play the role of confidante rather than rival. Her job also began to include tasks such as helping hedge fund entrepreneurs come up with compensation plans and track down general counsel or heads of investor relations. "Creativity, urgency, and execution—with Erin, you get the whole package," says Gerald Beeson, Citadel's C.F.O., who worked closely with her. "She's a real asset when you're tackling complex situations that require quick and clear thinking and fast action."

Callan is one of three daughters of a New York City police officer father and a homemaker mother. She attended Catholic high school in Queens and graduated from Harvard and New York University law school. She joined Simpson Thacher & Bartlett as an associate in the tax department, and Lehman happened to be a marquee client. She started nosing around for opportunities at the investment bank, which impressed her as a place where "everyone seems really into what they do." Lehman hired her initially to capitalize on her tax background in order to develop new kinds of investments for clients. She spent time in the fixed-income department before founding an internal think tank that designed esoteric financial instruments, a group that bankers could come to with their sticky client money problems. For example, if they were in the middle of an acquisition, with money trapped in France that needed to be moved to the Netherlands, Callan and her team would brainstorm for solutions.

Now, as an executive-committee member, Callan knows that she may someday be considered as a possible successor to Fuld. "She's brilliant, intensely focused, and you can rest assured, if it's on Erin's watch, she gets it done," says Gregory, Lehman's president. "I'm proud that Erin is our first woman executive-committee member, and I'll be proud of Lehman Brothers when she's one of many." But Callan demurs when asked about the role that gender might play in any future C.E.O. selection at the firm. "You know what? Dick [Fuld] is not going to care that you're one of the most successful women on Wall Street if he's not happy with the way you handled something," she says. "It's not really going to make a difference."

As for why there aren't more women like Callan—or even above or below her—she says that it's in part a numbers game. As the only woman on the executive committee, the most obvious pool of potential future C.E.O.'s, she has, at best, a 14-to-1 shot. A woman's slim chances and the dearth of female candidates, in turn, become mutually reinforcing: Since there aren't many senior women in finance, younger women don't go into the field.

Critical to Callan's success, she says, was having a network of peers and mentors—people she could bounce ideas off of, commiserate with, and who could provide a critical "sanity check" whenever she had a particularly conflict-ridden day or scarring political battle at the office. Without such people, she says, it's easy to be held back by self-doubt. "Probably the single biggest thing I had to get over in my career is not being liked by everybody and being okay with that," Callan says. "I think that's a challenge for a lot of women, when you've been socialized as a young girl and a woman to be polite and be liked."

Today, she has the unenviable task of taking control of the company's finances just as Wall Street finds itself in a state of crisis, coping with a hangover after a binge of easy credit and reckless risk-taking. She doesn't have an accounting background, which is increasingly key for C.F.O.'s as such regulatory requirements as Sarbanes-Oxley have upped companies' financial-reporting demands. But she's already trying to position herself as a big thinker rather than a bookkeeper.

"I see myself in more of the strategic camp," Callan says. "When you don't grow up in the accounting profession and you grow up more as a deal person, your bias is to look much more at the big picture." Otherwise, she says, "you really could consume yourself entirely just staying in the weeds."