For 20 years, Sundar Sundararaman has led a stream of well-intentioned Westerners into India's dark corners, bringing his often-wealthy guests face-to-face with AIDS-afflicted sex workers to drive home the depth of need.

"For many of them it's a big challenge to step out of the glass case," says Dr. Sundar, a mentor with Mysore and Mandya Direct Intervention, an organization that works to stem the spread of HIV.

About two months ago he welcomed Melinda French Gates, a woman whose own guest list – at the earth-sheltered lakefront mansion in Medina, Wash., that she shares with her husband, Microsoft multibillionaire Bill – has included the premier of China.

"As an onlooker, I was taken aback," says Sundar. Even away from the cameras Ms. Gates had an easy rapport with "the marginalized," he says, the drug-addicted and the transgendered. "She was engaged in asking very specific questions about whether this project was touching their lives.... There was a natural person in her, an individual who connects with people."

It is a selective kind of connectedness. The enigmatic Gates – her interviews famously rare, her close associates reticent, her three young children shielded – could be considered the anti-Angelina Jolie in her approach. Gates – by all accounts an active partner in the Bill & Melinda Gates Foundation – is, as a Fort Worth, Texas, paper recently declared, "the most powerful woman you know next to nothing about."

Her clout has begun to swell of late, and along with it public curiosity about her. Philanthropist (and close family friend) Warren Buffett's recent record-setting $31 billion gift to the foundation heightened the mystery around the woman whose face recently was book-ended by those of Mr. Gates and rock star Bono on the cover of Time – but was unrecognizable to all but four of 20 Seattle-area residents shown her photo, without context, by the Monitor.

"There are many other people in Seattle who make sure they're seen at all the big events," says Dottie Simpson, wife of the late Seattle philanthropist W. Hunter Simpson and a longtime friend of Bill Gates's parents, William H. and Mary. "That's not Melinda's style."

A spokeswoman for Gates Foundation rules out an interview in a month largely set aside for family. "When they're off," says Amy Siegel, "it's very guarded time." A portrait emerges slowly from scattered clues among Gates's public statements and from tales coaxed from those just outside the formidable cone of silence.

A distance-runner and kayaker, she admits to bouts of stage fright. She considers the late independent and tenacious Washington Post grand dame Katharine Graham a role model. Gates reportedly had her activist's epiphany when, while on safari in Africa in 1993, she watched women trekking shoeless for miles in the dust to sell a few vegetables.

"Bill and I believe one life is worth no more or less than any other," Gates told the Times (London) last year. "That issue of equity is what the foundation stands for."

In another sense, greater equity could be key to the foundation's success. It could call for Gates to shed the relative anonymity she has nurtured since she met Bill Gates at a company event in New York in 1987. It's a step out that she indicated in a 2002 Newsweek interview she'd probably try if it would help advance the development of an AIDS vaccine.

"Women often do not claim their power and put their whole identity out there in the public so that they become the engine," observes Swanee Hunt, the former ambassador, philanthropist, and director of the Women and Public Policy Program at Harvard's Kennedy School. "I think it will be great as she increases her voice."

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But Gates ispracticed at insularity. A Microsoft employee from 1987 to 1996 (developing multimedia products), she often ate alone in the company cafeteria after her 1993 engagement to the company's founder because she was too intimidating a lunch date, recalls a former Microsoft employee who worked under her at the time but asked not to be named.

"It was a tough spot to be in, an employee engaged to Bill Gates," says the ex-employee, who found her boss to be bright and pragmatic, if a little aloof. "When I say she reminded me of a Catholic schoolgirl, I mean it in the best possible way," she says. "She had a real sense of humanitarian virtue."

Not that her success hasn't rankled. Gates's e-mail announcing her departure from the firm cited the tug of war between family and work, says Rebecca Hughes, another co-worker of Gates at the time. "For me, I felt sad," says Ms. Hughes, "because I was playing that same tug of war with my son and my work, but I didn't have the luxury of quitting."

Gates still appears to wrestle with establishing normalcy. The Gateses occasionally venture out to movies and parks. Melinda is said to have softened a house some initially described as having the aura of a corporate retreat. As Bill Gates told Newsweek in 1999: "Melinda was saying 'Maybe we shouldn't move in, because [it may not] really feel like a home is supposed to feel.' "

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Born in Dallas in 1964 to Raymond (an engineer) and Elaine Amerland French, Gates attended the all-girls Ursuline Academy – school motto, "Serviam." She often credits teacher Susan Bauer with putting her on a path to math and computers.

Gates earned a bachelor's degree at Duke University, in Durham, N.C., in 1986, and a Duke MBA the following year. Rich Burton, professor of management at Duke's Fuqua School of Business remembers Melinda French, a student in his management-control class, as reserved, "very serious" – and insightful.

"As an instructor you make up a key of model answers," Professor Burton says. "I was grading [exams] and I came to one on which the answers were better than mine," he says. "That was Melinda's. I started using it as a key."

Another professor, Alan Biermann, remembers being struck by Gates's ability to advise without flaunting her intellect. In the late 1990s, Gates visited his computer-science class. A student described the difficulty he was having making a tabletop robot "see" variations in shading of color-coded blocks it was programmed to move.

"Melinda was quite interested," he recalls. "And then exclaimed how she now understood why the robot lab at Stanford had the very uniform soft lighting that she had seen there."

Gates has been graceful, too, at the receiving end of guidance. Mary Gates reportedly wrote a letter to the couple before their 1994 wedding about the responsibilities that come with wealth. Today, the money is moving: more than $1.3 billion in grants in 2005. The pair has reportedly promised to give away 95 percent of their wealth in their lifetimes. "I'm so pleased with what the Gateses have done with their funds," says Mrs. Simpson, adding in reference to the Buffett bequest, "I think it's catching on."

Renowned for her arcane knowledge of diseases, Gates also exudes business savvy. "Although [international governments'] funding increased from $65 million in 2000 to $163 million in 2005," she wrote in a Newsweek article in May, "current spending is only about half of what is needed to advance the most common microbicide candidates." It is classic Melinda: specific, unflashy, quietly passionate.

"I have met with a lot of influential people," says Sundar. "[Gates] has a very unique ability to comprehend what she sees and hears." What sets her apart, he says, is that she responds to need in a way that is personal and direct. "That simplicity," he says, "is what is inside her."

• Dean Paton contributed from Seattle.