That man is Sidney Blumenthal. According to the Huffington Post's Sam Stein, in the first session alone Thursday, he was mentioned exactly as many times as the U.S. ambassador who died in the Benghazi attacks.
Blumenthal's name came up again and again because his e-mail address filled Clinton's e-mail inbox again and again, before and after the attacks. Between 2011 and 2012, he sent Clinton at least 25 memos consisting of intelligence and strategy reports, advice, articles and even gossip. His memos filled about a third of the nearly 900 pages of e-mails related to Libya released from Clinton's private e-mail server.
Here are the bullet points of who he is and what you need to know about him:
Full name: Sidney Stone Blumenthal
Goes by: Sid
Biographical info: Age 66, Chicago native, lives in D.C. with his wife, with whom he has two sons
Connections to the Clintons: Blumenthal got his start as a journalist, and he first covered Bill Clinton when he was governor of Arkansas in the 1980s and later his presidential campaign and early years of his presidency. They became friends and, in 1997, Blumenthal went to work for Clinton, where he stayed until 2001.
What he did in the White House: As a senior adviser, Blumenthal used his media experience to form a communications strategy for Clinton. He was a fiery character willing to defend Clinton at all costs, according to accounts.
In 1998, Washington Post reporter Michael Powell wrote: "When the White House is backpedaling – a familiar move as the president's libido made a hostage of his administration – Blumenthal is the first to urge aides to man the ramparts."
Blumenthal also was known for sometimes weaving conspiracy-like theories. The Washington Post's Karen Tumulty said his nickname while in the White House was "The Grassy Knoll," a reference to the most famous of the JFK conspiracy theories.
This isn't the first time Blumenthal has played a role in a major political inquiry: Speaking of the Monica Lewinsky scandal, Blumenthal testified to a grand jury about what Clinton had said to him and other senior staff members about the intern. Blumenthal was also one of four witnesses who testified to the Senate during its impeachment trial of Clinton, where he spoke on whether he had passed on rumors about Lewinsky to the media.
He's also an author: Blumenthal has written about half a dozen political books. Back in 1980, Blumenthal helped popularize the phrase "permanent campaign" with his book titled just that. He made the case that politicians spend more time trying to get reelected than governing.
Another notable book is 2003's "The Clinton Wars," an 800-page memoir of his time at the White House that the New York Times called "a welcome addition" to the volumes about Clinton. It came out one year before Clinton's autobiography. In writing so much about how Clinton's public scandals consumed the White House, the New York Times said Blumenthal's book also served as "a kind of lawyer's brief for the accused."
There's another book coming in May from Blumenthal: "A Self-Made Man," pitched as "the first of a multi-volume history of [Abraham] Lincoln as a political genius."
He worked on Hillary Clinton's 2008 campaign: And when she got the secretary of state job, the ever-loyal Clinton tried to bring him on as her adviser. But she ran into trouble with the Obama administration, which was upset with Blumenthal's attacks on Obama during the primary and nixed the idea.
About his Libya e-mails to Clinton: Blumenthal instead got a job with the philanthropic Clinton Foundation. He also advised business interests in Libya at the time he was e-mailing Clinton, which raised red flags for Republicans investigating Benghazi.
Back in June, Blumenthal gave his own marathon testimony to that House panel investigating Benghazi. He spoke to the committee for more than eight hours and later told reporters that he said he did not write the intelligence memos he sent to Clinton -- he simply forwarded advice on from a former high-ranking CIA official to use as she saw fit.
On Thursday, Clinton told the committee she did exactly that.
"He sent me information he thought might be of interest," she said. "Some of it was, some of it wasn't, some of it I forwarded to be followed up on. The professionals and experts who reviewed it found some of it useful, some of it not."
Republicans tried to draw a contrast between Blumenthal's nearly unfettered access to Clinton and the communications of then-U.S. Ambassador to Libya Chris Stevens, who in requesting greater security in Benghazi didn't have access to the private e-mail address Clinton used almost exclusively as secretary of state. Benghazi select committee Chairman Rep. Trey Gowdy (R-S.C.) also read aloud e-mails Blumenthal allegedly wrote disparaging the secretary of defense and other top Washington figures.
Clinton's reply to Blumenthal's stature: "You know, Mr. Chairman, if you don’t have any friends who say unkind things, I congratulate you."
She also maintained Blumenthal had no official position advising her on Libya.
The Washington Post's Fact Checker, Glenn Kessler, backed up that assertion when on Tuesday he rated as false Republicans' suggestions that Clinton relied mostly on Blumenthal for her intelligence on Libya.
Reading through the Blumenthal e-mails, posted on the State Department Web site, one sees that this is mostly gossip of dubious value," Kessler wrote. "Sometimes Clinton would forward it on to an aide, sometimes not."
Democrats on the panel were not happy with the focus on Blumenthal.
But as long as the Benghazi investigations keep going, you can expect to keep hearing his name.