Bill Gates is worth an astounding $81.6 billion and he keeps getting richer every year.

His secret weapon is a man you have probably never heard of: Michael Larson.

Gates hired Larson 20 years ago, when his net worth was a relatively paltry $5 billion, report Anupreeta Das and Craig Karmin at the Wall Street Journal, who just wrote a profile on the notoriously secretive Larson.

Larson runs Gates' personal investment company Cascade Investment LLC, funded solely by Gates.

At one time, Gates' wealth depended solely on Microsoft. But for years he's been selling off his Microsoft stake. The common perception is that he's been using the proceeds from those sales directly for charity. That's not entirely how it works.

Although Gates makes his own investments in tech, it is Larson, through Cascade, who has taken Gate's money and diversified it. Gates now has vast holdings in real estate and nontech companies like the Canadian National Railway Co., AutoNation Inc., and Republic Services Inc. It is these vast holdings that help fund the Gates' donations.

And although Gates has given an astounding $38 billion to his charitable foundation, thanks to Larson, he's getting richer faster than he can give his money away.

His $81.6 billion is nearly $6 billion more than it was as of March 2014, when he was worth $76 billion, we reported at the time. And $76 billion was $9 billion more than he was worth in March, 2013.

Bill Gates. REUTERS/Rick Wilking

In February, Gates celebrated 20 years of this partnership by throwing a gala to honor Larson at his Seattle mansion, reports the WSJ. It was a rare occasion where the two men socialized with each other. Apparently, they aren't buddies and don't hang out much, sources told the Journal.

At the party, Gates told guests he has "complete trust and faith" in Larson, meaning that Larson invests Gates' money, buying and selling, with complete autonomy.

And he does it all under a cover of such ferocious secrecy that he's been nicknamed "the Gateskeeper."

Although publicly traded companies do reveal when Cascade has invested heavily in them, Larson has all sorts of tricks for keeping Cascade's and Bill Gates' name out of other investments, sources told the Journal.

For instance, he makes employees sign confidentiality agreements which cover them even after they leave. He farms out more than $10 billion to up to 25 outside money managers. This helps him find new investment ideas, but it also helps cover the trail. When Cascade was part of an investment group that bought the Ritz-Carlton hotel in San Francisco, the publicist didn't even know Cascade, and Bill Gates, was among them.

He's also been known to fire up a limited liability corporation to make real estate purchases, to keep Cascade's name off the deal and the deed.

He's so good at hiding the trail that most people don't know that Gates, through Cascade, owns a significant stake in the Four Seasons luxury-hotel chain.

And he's frugal with the bosses' money, too. Apparently, Cascade employees, of whom there are about 100, are not allowed to stay at the Four Seasons when traveling on business, even if that business is on behalf of the Four Seasons. They must choose a lower-cost, less luxurious hotel.

"Melinda and I are free to pursue our vision of a healthier and better-educated world because of what Michael has done," Gates told guests at the party.

And ultimately, the money will go to charity. Bill and Melinda Gates have vowed to donate 95% of their wealth to their foundation, as part of Gates' giving pledge.

But until then, Larson is making it grow.