Who is Steve Ballmer, the new chief executive of Microsoft?

He's big, loud, funny and fearless. He's a hardball negotiator who never seems to give up or tire.

And he's definitely not diplomatic. Soon after the Department of Justice filed its massive antitrust case against the company, Ballmer blurted out, "To heck with Janet Reno!"

Though he later retracted the comment, the line became a symbol of Microsoft's attitude toward the government's attempts to rein it in.

"Steve has been the heart and soul of Microsoft as long as he's been there," said Nathan Myhrvold, Microsoft's chief technology officer, who calls Ballmer "absolutely one of the key contributors" to the company's phenomenal success.

That's why Microsoft co-founder Paul Allen says that Ballmer's promotion within the company that made him a billionaire more than 20 times over is "a natural evolution of Steve's responsibilities."

To anyone who has not followed Microsoft closely, however, Ballmer is an unknown--especially compared with the hyper-famous Bill Gates.

Until recently, Ballmer has been "eclipsed by the Bill Gates press persona--he hasn't been written about near as much as he deserves," said Myhrvold, currently on a one-year leave from the company. "A lot of coverage of Microsoft tends to focus on Bill as if he did it all single-handedly!"

Eclipsing the explosive, expansive Ballmer is no small feat.

"I've always thought Steve was a force of nature," Myhrvold said. "He is an incredibly talented guy, he is an incredibly passionate and sincere person all rolled into one. . . . He's got more energy than anyone I have ever known."

Ballmer, 43, grew up in Detroit, the son of immigrants, a Swiss father and a Russian mother. Ballmer's father worked his way up to middle management at Ford Motor Co.

Ballmer made his way to Harvard University, where he met an undergraduate by the name of Bill Gates in 1973. They hit it off, but Gates got involved in some computer thing and dropped out.

Ballmer was a total-immersion Harvard man, managing the football team and working with the Harvard Crimson and the literary magazine. He got his degree.

Ballmer reunited with his college pal by joining Microsoft in 1980, after working at Procter & Gamble as a marketer of cake mixes and later attending the Stanford business school.

At the embryonic software company, he helped bring discipline and spirit. He became the top internal cheerleader, a loud one. His high-volume exhortations came at a price, however: He had to undergo surgery to repair the damage he has done to his throat over the years.

In 1998, Gates appointed Ballmer president of Microsoft, giving him responsibility for day-to-day operations.

Ballmer is the closest thing Gates has to an alter ego. They share a hard-driving approach to competition and to management, with Ballmer's outgoing nature a strong complement to Gates's social awkwardness.

"Ballmer can be even more outspoken and hard-charging than Gates," said Brian Goodstadt, a technology analyst at Standard & Poor's equity group in New York.

True to form, Ballmer minced no words today on the topic of Microsoft's threatened breakup after a landmark suit by the Department of Justice.

"I think it would be absolutely reckless and irresponsible for anyone to try to break up this company," Ballmer said. "I think it'd be the biggest disservice anyone could do to consumers in this country."

As energetic outside the office as inside it, Ballmer is a daily jogger and avid basketball player. He does not live the high life of some high-tech zillionaires--except, perhaps, for that stake he owns in the Seattle Sonics basketball team. He is married, to a former Microsoft employee, with three children; he speeds home most nights to put the kids to bed.

With Ballmer in the top job, the pace of the fever-pitch company could actually speed up even more. "Steve has his own management style," co-founder Allen said. "He's a very no-nonsense, pragmatic, dynamic, hands-on manager."

At the news conference yesterday, Ballmer and Gates said the company will continue in the direction of putting its software on every possible kind of device--not just personal computers and Web-equipped televisions, but hand-held organizers, cell phones and even kitchen appliances--and tie it all together via the Internet.

Ballmer rhapsodized geekishly: "We need to deliver a breakthrough version of Windows that allows the PC and servers to include these next-generation services and to use those services actually hosted out on the Internet. . . . And it's imperative that the breakthroughs that we're talking about here include changes to the programming model, to the user interface, the application integration model, the file system, new XML schema."

Bob Frankston, the co-developer of VisiCalc the software that became one of the first "killer apps" of personal computing, worked with Microsoft, where he got to know Ballmer's "Crazy Eddie impersonation."

Frankston chuckled in a conspiratorial way and said, "It's an act!"

In fact, Frankston said, Ballmer is "a reasonable person to talk to," with no small measure of friendliness and warmth. "You can't tell about that," Frankston said. "It's a secret."

On the other hand, Frankston admitted, his relationship with Ballmer is informal. "If I had him across a bargaining table, I'd have different feelings," he said.

IN PROFILE: Steve Ballmer

Position: Microsoft president and CEO, responsible for overall management of the company.

Age: 43

Education: Bachelor's degree in applied math and economics, Harvard University (where he met Microsoft's founder, Bill Gates); attended Stanford Graduate School of Business (but left when Gates persuaded him to join Microsoft).

Career highlights: Assistant product manager at Procter & Gamble after graduating from Harvard; in 1980, joined Microsoft, where he has held various positions including executive vice president of sales and support, senior vice president of systems software, and vice president of marketing.

Other: Ranked fourth among Forbes's wealthiest Americans in 1999.

SOURCES: Microsoft, Who's Who, Bloomberg News

CAPTION: Steve Ballmer, Microsoft's new chief executive, is known for his hard-driving style.

CAPTION: New chief executive Steve Ballmer, left, with Chairman Bill Gates yesterday at a news conference at Microsoft's headquarters in Redmond, Wash.