Microsoft executives talk about Ballmer's ability to digest large chunks of data, while carefully probing business proposals for weaknesses in logic or reasoning.
Ballmer's sales and marketing prowess complemented Gates's technical acumen as Microsoft grew from a fledgling start-up into a world-beating software company.
He worked up the ranks, becoming Microsoft's president in 1998 and replacing Gates as CEO in 2000. Ballmer is Microsoft's second-biggest shareholder after Gates with a 4.3 percent stake in the company, valued at more than $11 billion.
Michael Silver, analyst at research firm Gartner, describes Ballmer's management style as "scary," but credits him for being a good listener to the needs of his customers.
"Steve's a bright, tough guy and a good marketeer," said Silver. "His personality can be very imposing."
Ballmer often grabs headlines with sharply worded jabs at competitors. He once called free Linux software "a cancer" and dismissed Web search leader Google as "a one-trick pony."
His exuberance for all-things Microsoft has also earned him viral video fame on par with lonelygirl15 or Obama Girl. Video of Ballmer's enthusiastic support for software developers has been viewed more than 1 million times on YouTube, a performance that earned him the unflattering nickname of "Monkey Boy."
"He was always the foil to Gates," said Mary Jo Foley, author of "Microsoft 2.0: How Microsoft Plans to Stay Relevant in the Post-Gates Era."
"Gates is such a serious, plodding, methodical guy and Ballmer knew that to be part of the dynamic duo with Bill, he needed to be the opposite."