Ruth Turner, who has been arrested and questioned in connection with the cash-for-honours inquiry, is one of Tony Blair's closest advisers.
Ms Turner was a successful publisher and fundraiser
As Downing Street's director of government relations, she is at the heart of government organisation.
But the 36-year-old is not considered a political careerist, having enjoyed success in fundraising and publishing.
Her current job is to ensure smoother communications between ministers and MPs.
She also has the key role of running Mr Blair's diary.
However, her achievements go far wider than working in government.
Before taking on the director of relations job after Labour's 2005 election victory, Ms Turner was perhaps best known for launching the Big Issue magazine in north-west England in 1992.
Still in her 20s, she formed a Manchester-based social research company, which ran "citizens' juries'' to come up with ideas for improving communities.
This is a tactic currently popular with Mr Blair and his fellow policy makers.
Ms Turner later worked in fundraising for the National Endowment for Science, Technology and the Arts.
She was also seen as a moderniser when she sat on the board of the New Local Government Network think tank.
And in 2000 she gained a place on Labour's National Executive Committee and has stood for Labour in a European election.
Ms Turner is known for having an easygoing style and has featured far less often in the headlines than her predecessor, Anji Hunter.
In 2002 she joked to North West Business Insider magazine: "I hadn't really any intention of joining the Labour party, I just fell in with the wrong crowd."
Ms Turner, who is unmarried, is a close friend of Labour peer and film producer Lord Puttnam, who told the BBC: "She's one of those half-dozen, dozen people who I would stake my life on."
Ms Turner, who was released without charge following her questioning by police, denies any wrongdoing.