Who is this majority shareholder standing guard against the next potential mega-merger in the global beer market?

Her name is Charlene de Carvalho-Heineken. The only child of the late Freddy Heineken, who brought the premium beer brand to America after Prohibition, she is the sole heir to the Heineken fortune and the controlling shareholder of what is now the world’s third-largest brewer.

De Carvalho-Heineken, 60, controls 51% of Heineken Holding NV (HKHHF), a public company listed on the Amsterdam Stock Exchange, which in turn owns 50.05% of Heineken NV (HINKF), also listed on the Amsterdam exchange. The stock-market value of Heineken NV is around $45 billion, making de Carvalho-Heineken’s interest in the company worth more than $11 billion. She is one of Europe’s wealthiest women and one of the world’s least known billionaires.

Shy and extremely private, de Carvalho-Heineken has never spoken to the press—until Fortune spent two days with her and her husband, Citigroup (C) vice chairman Michel de Carvahlo, in Amsterdam in June. “I’ve always been quite happy to be anonymous,” she told me.

It was de Carvalho-Heineken’s great-grandfather, Gerard Adriaan Heineken, who founded the company 150 year ago, and her father, Freddy, who expanded Heineken globally with ferocious drive and an iron will. Charlene grew up in Amsterdam, sheltered by and very close to her father. After Freddy was kidnapped in Amsterdam and held hostage for a large ransom in 1983, Charlene spent her life happily below the radar in London, raising five children with her banker husband. Michel de Carvalho, a former child actor and Olympic skier, is as outgoing and effusive as Charlene is quiet and reserved.

She joined the board of Heineken Holding NV in 1988 but had little interest in the details of the family business until 12 years ago. That’s when her father died. On the day of Freddy’s funeral in 2002, Michel, playing investment banker as well as consoling spouse, told Charlene that it would be prudent to have a Heineken involved in the company. With trepidation, she stepped up.

Digging into Heineken’s global operations and visiting breweries around the world, the Heineken heiress and her husband, 70, turned into very involved and opinionated owners. They played key roles in replacing Heineken’s CEO in 2005 with Jean Francois van Boxmeer, who is now at the helm. Under the couple’s watch, van Boxmeer acquired brands such as Dos Equis and Bohemia—while fending off suitors who would relish owning these and other Heineken brands such as Amstel.

Neither de Carvalho-Heineken nor her husband, who is on the supervisory board of Heineken NV, agreed to talk to the press about the takeover overture from London-based SABMiller—which is looking to bulk up for its own protection against potential acquisition by behemoth Anheuser-Busch InBev. During two days of interviews in June, though, the de Carvahlos talked candidly about the threat of takeover—“I worry about everything,” Charlene admitted, noting that she was “very, very, very sorry” when August Busch sold his family business, Anheuser-Busch, to Brazil-based InBev in 2008. “That was shocking,” she said.

“The lady who throws the switch,” as Michel deCarvalho calls his wife, will continue to say ‘No’ to takeover bids in the future. “It’s not sentimentality,” said Michel. He and Charlene intend to pass on their controlling stake in Heineken to their five children. Charlene’s late father, Freddy, called this fourth generation of Heineken the “D-generation”—jokingly, perhaps—and Michel and Charlene want to prove that the title doesn’t fit. “The best defense is always a high stock price,” Michel says, adding, “With our strong culture built up over 150 years and a workforce as passionate as we are, we’ll drive this company forward with excellent returns for all shareholders.”