He's always there, operating just below the radar.
Jonathan Powell, Tony Blair's chief of staff, has been heavily involved in the preparations for this week's Leeds Castle talks.
But don't ask him what he's doing - he won't tell you.
The former Downing Street press officer Lance Price says Mr Powell's "complete and absolute discretion" is crucial to his role in trying to reconcile the different parties in Northern Ireland.
"All sides in the debate know they can trust Jonathan utterly.
Mr Powell advises the PM on everything from Iraq to the euro
"Anything they say to him will be kept in confidence, that they can therefore be sure that there's no danger of their positions being compromised, and also beyond that, of course, he has the ear of the prime minister.
"He can speak to Tony Blair at anytime of the day or night, and therefore all the parties know that not only will anything they say be treated with confidence, but also it will get to the prime minister if it needs to."
So imagine how embarrassing it was for this discreet official when the transcript of a bugged phone conversation revealed him joking with Sinn Fein's Martin McGuinness about Northern Ireland politics, and calling former Ulster Unionist MP Willie Thompson "an ass".
Undiplomatic language, but Martin McGuinness won't have a bad word said about him.
Two Powell brothers
The Sinn Fein negotiator says the Downing Street chief of staff is "clearly a very experienced diplomat".
"He's personable, he can enjoy a joke just like the rest of us, and I think he can be just as serious as the rest of us when it comes to dealing with these critically important issues," he said.
Working at Number Ten runs in the family - Jonathan's elder brother Charles was Margaret Thatcher's right hand man.
By contrast, Jonathan teamed up with Tony Blair, who he met when he was a Foreign Office diplomat, detailed to brief the then opposition leader on Bill Clinton's rise to power in the United States.
The two men hit it off, and Powell left the civil service to work for the Labour Party.
That's not the only contrast between the two Powell brothers.
Peculiarly, Charles prefers his surname to be pronounced like the word "pole", while Jonathan chooses the more usual rendition rhyming with "owl".
Jonathan Powell advises Tony Blair on everything from Iraq to the euro.
'Merely an aide'
But Northern Ireland is a special subject - he worked around the clock on the Good Friday Agreement, and chaired talks intended to defuse the Drumcree parade dispute.
Some say he is the real Northern Ireland secretary - others beg to differ.
Ulster Unionist leader David Trimble says Powell is "the prime minister's gofer".
He describes him as "merely an aide" and claims "it's a bit of a mistake to assume that he personally has any role in terms of policy making".
But the DUP's Nigel Dodds disagrees. "There's no doubt that the secretary of state plays a very vital role," says Mr Dodds.
"But Downing Street takes a much more hands-on approach, and since the prime minister can't be directly involved in a lot of meetings on a day to day basis, then obviously he delegates that to Jonathan Powell.
"So I think Powell is a very, very
important figure ."
Martin McGuinness concurs. He says Paul Murphy "deals with certain mundane matters on an ongoing daily basis".
"But if you are talking about who takes the big decisions in relation to the situation of this part of Ireland from a British Government perspective, I have to say that it's Ten Downing Street and it's Jonathan Powell in conjunction with Tony Blair."
Since the Good Friday Agreement, secretaries of state have come and gone.
But the Downing Street chief of staff has remained at the centre of the web, despite some rumours that he might seek a career outside government.
If Tony Blair and Bertie Ahern emerge from Leeds Castle clutching a piece of paper which they believe spells peace in our time, one thing is certain - Jonathan Powell's fingerprints will be all over it.