Adam Werritty has played a big part in the life of Defence Secretary Liam Fox - his best man, a former flatmate and a one-time business partner.
Now, Mr Fox finds questions around their working relationship have ended his tenure as defence secretary.
The personal and business ties between the two run deep.
They first met in the late 1990s when Mr Fox was an opposition spokesman on Scotland and Mr Werritty was studying public policy at Edinburgh University.
Their friendship grew, despite a 16-year age gap, around a shared interest in politics and the US.
In 2005, Fife-born Mr Werritty was by Mr Fox's side when he married Jesme Baird at a Westminster church.
He was also director of Atlantic Bridge, a charity founded by Mr Fox to promote economic and cultural links between the US and UK, which was dissolved last month.
More recently he met the defence secretary on 18 overseas trips, brokering a meeting in Dubai with a potential defence contractor, despite neither working for the government nor the Conservative party. Among those trips were two family holidays.
Mr Werritty, 34, who lives in London, carried business cards embossed with parliament's portcullis logo and described himself as an "adviser to the Rt Hon Dr Fox MP".
The Times claimed this self-styled role went as far as having a reputation among lobbyists as the "go-to guy" for gaining access to the defence secretary.
Mr Werritty, who has run health and defence-related businesses in the past, also visited Mr Fox's office at the Ministry of Defence 22 times in a year-and-a-half - which prompted Labour to call for an inquiry into whether national security had been breached.
There have been questions about how his overseas trips were funded. Mr Fox said Mr Werritty was "not dependent on any transactional behaviour to maintain his income".
The Times has reported that his financial backers include a private intelligence company with strong interests in Sri Lanka and a property investor who lobbies the UK government on behalf of Israel.
The newspaper said they helped channel £147,000 into a not-for-profit company set up by Mr Werritty, who used it to join the defence secretary on trips abroad.
Mr Werritty has now been questioned twice as part of the inquiry headed by the Cabinet Secretary Sir Gus O'Donnell.
Mr Fox has admitted allowing the lines between his personal friendships and his government activities as defence secretary "to become blurred".