Yahoo has been short on friends this year. Once it caught the attention of industry bully Microsoft, the company fruitlessly sought help from Google, Time Warner's AOL, and even News Corp.'s Rupert Murdoch—who, it turns out, likes the bully better.

But one true believer has faithfully stood by them, even doubling its stake in Yahoo to more than 10 percent of the company. Yahoo said on Thursday that Capital World Investors of Los Angeles had upped its stake in the company to 136 million shares at the end of March from 70 million shares three months earlier.

In December, Capital World and Capital Research were split into separate divisions because the combined entity had grown too unwieldy. But because both manage money in the same family of funds, it's possible the total stake in Yahoo could prove even larger once Capital Research reports its own stake.

"You can't assume the Capital World numbers reflect a diminished holdings on the other side," said Chuck Freadhoff, a spokesman for Capital Group. "All I know is we didn't file a Capital Research filing today on Yahoo."

Often, when a major investor boosts its shareholdings as a takeover grows hostile, it's someone angling for a board seat so he or she can play activist. But this time, the aggressive investor clearly has a different game plan.

Who is this secret admirer of Yahoo? The Capital Group is among the largest investors in the United States, but the average retiree may know it better by the brand name of its flagship fund family, American Funds. It oversees 50 million shareholder accounts and more than $1 trillion in investments, which is up from $360 billion in five years.

American Funds has leaped into the ranks of Fidelity and Vanguard in recent years, quietly so because it's bypassed mass marketing to work with investment advisers. "The advisers in general have been big fans," said Greg Carlson, a Morningstar analyst who follows the Capital Group. "They like the stability and their long-term approach."

Carlson says that approach is grounded in an unusual practice in the industry: deploying several managers for each mutual fund. That makes it harder for one manager to stand out as a star—or to crash and burn after reaching for stellar returns.

So American Funds hardly fits the profile of a Microsoft stooge, or even a closet corporate raider. Which makes its interest in Yahoo that much more intriguing. What is its motive?

Past filings for Capital World Investors' other recent investments turn up an interesting pattern. In January, both Capital World and Capital Research co-invested in Citigroup with Sandy Weill and Prince Alwaleed bin Talal, right when the market was down on Citi.

And in February, Capital World disclosed a similarly contrarian 6 percent stake in Countrywide (but unlike fellow Yahoo investor Legg Mason, it seems to have built up its ownership after Countrywide's collapse).

The thinking of Yahoo's biggest fan seems to be this: Bring us your downtrodden, your underdogs, your lowly valued. If they are right, then they're likely betting that Yahoo will end up selling at a higher price from here.

Either Yahoo will buy its shares back above the $29 a share Microsoft has proposed, or Microsoft will. Given all the recent machinations and corporate politicking behind this deal, it's probably not a bad bet.