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"So you think you gave a historically significant speech, huh?" Valerie Jarrett teased Barack Obama after he delivered his well-received address on race, in Philadelphia. During the speech, Jarrett sat next to Michelle Obama, the two old friends promising each other they wouldn't cry. "Ten minutes in, and we just looked at each other, and the tears started flowing," Jarrett says. But in the greenroom afterward, she was back to ribbing the Illinois Senator on the comparisons already being made to Martin Luther King Jr. and Bobby Kennedy.

Jarrett, 52, is that kind of friend to the Obamas--she can tease and cry and in the next moment weigh in on policy matters. She has been called the dean of Barack's kitchen Cabinet, but her role has changed over their 17-year friendship: first as Michelle's boss at Chicago's city hall, later as finance chair for Barack's Senate campaign.

In her day job, Jarrett runs the Habitat Co., a real estate firm, and she chairs the board of the University of Chicago Medical Center as well. She has also served as chair of both the Chicago Transit Board and the Chicago Stock Exchange. But as Jarrett sees it, her most important position may be the role of honest critic for the man she hopes will be President. "I'm very frank, and I always tell them what I think," says Jarrett. "But that's probably easier to do when you're good friends."