Twitter may be having one of the highest-profile initial public offerings in years, but its single largest outside shareholder has almost no online profile.

It's a firm called Rizvi Traverse, and its funds own 17.9 percent of Twitter pre-IPO, 15.6 percent after. It's a bizarre juxtaposition, considering that Twitter's business is based on people's communicating publicly.

But many sources say that Rizvi's low profile helped solidify its relationship with Twitter and secure its access to the company, as well as to other hot start-ups.

Rizvi and its funds have a stake that will be worth $1.7 billion if Twitter's shares are priced at $20—the top of its target range. The firm started investing in the social network at the end of 2010, and its funds have since drawn big-name investors, including JPMorgan, which through its investments will own a 9 percent post-IPO stake in Twitter.

Saudi Prince Alwaleed bin Talal also invested in Twitter through Rizvi, and his Kingdom Holding owns more than 20 million shares.

For a man whose firm controls such a major stake in a such a talked-about IPO, Rizvi Traverse co-founder and Chief Operating Officer Suhail Rizvi stays amazingly below the radar.

There are no public photos of him—he reportedly hires people to scrub the Web of pictures and to monitor his Wikipedia page. The only public data we could find is a website describing "a value-oriented, opportunistic investment strategy that combines capital with management expertise," and lists Rizvi investments.

(Read more: Why Twitter's IPO should be more like Facebook's)

Rizvi Traverse owns a controlling interest in Playboy and music rights organization Sesac, as well as stakes in news app Flipboard and Jack Dorsey's digital payments company, Square. Sources told CNBC that Rizvi invested $100 million in Facebook before its IPO and sold its shares earlier this year. It also has sold its equity stake in talent agency ICM, and in "Twilight" producer Summit Entertainment, which sold to Lionsgate.

Rizvi's Twitter connection, which led to its Square and Flipboard investments, originated with Chris Sacca of Lowercase Capital. An angel investor and Twitter advisor, Sacca has been building Twitter-related funds since 2008, bringing in investors such as Ashton Kutcher and Richard Branson.

Sacca, who was previously head of special initiatives at Google, manages a portfolio of more than 50 start-ups, including car service Uber, Kickstarter and Blue Bottle Coffee.

(Read more: Inside the NYSE's systems test before the Twitter IPO)

At the end of 2010, Sacca reached out to his friend Rizvi about Twitter and introduced him to management. Though Sacca's funds own significant chunks of Twitter (they carry opaque names like Institutional Associates fund, with 32 million shares) neither his name nor Lowercase Capital are listed in Twitter's S-1 filings.

Both Sacca and Rizvi Traverse agreed to be long-term holders of Twitter, not selling until at least 180 days after an IPO. Sacca has been quiet lately, though he is a well-regarded voice in Silicon Valley on various lists of influential and savvy investors.

Rizvi is taking more of a public approach than it has in the past, hiring a PR firm to address the inevitable flood of calls about its Twitter stake.

We'll see if the public sale of Twitter shares prompts Rizvi to start talking.

—By CNBC's Julia Boorstin. Follow her on Twitter: @JBoorstin.