London liberal incomprehension, strong in so many areas, is in few so comprehensive as on the subject of Arlene Foster, leader of the Democratic Unionist Party.

It would be surprising if one person in a hundred who follows Westminster politics could give an adequate account of her.

Not that Foster, DUP leader since December 2015, has done much to explain to the wider world what kind of person she is. She is certainly not a second Ruth Davidson, ready to spring before the public as the very model of a modern party leader.

And yet Foster does symbolise a major change in the DUP, which after this year’s general election reached a confidence and supply agreement with the Conservatives under which its ten MPs support the government in important parliamentary votes in exchange for a billion pounds of additional public spending in Northern Ireland.

Foster is the first woman to lead the DUP, and the first Anglican, taking over from Peter Robinson, leader from 2008 to 2015. The party was founded by Ian Paisley, who led it from 1971 to 2008, and was also the founder in 1951 of the Free Presbyterian Church of Ulster.

Although Paisley ended up sharing power with Sinn Fein, and on the best of terms with Martin McGuinness, he made his name as an intransigent sectarian opponent of all concessions to Irish nationalism and the Church of Rome. The fervour of his denunciations, which in the 1970s encompassed the destruction of power-sharing and a campaign to Save Ulster from Sodomy, appalled not just English liberals but respectable Ulster Unionists, whose leader, David Trimble, he at length managed to elbow aside.

Hot gospelling is not Foster’s style. She is, however, deeply marked by her early experiences of the Troubles. Her father, a part-time officer in the Royal Ulster Constabulary, also ran a small, remote farm, in a nationalist district close to the border with the Republic of Ireland in County Fermanagh. Foster described in The Belfast Telegraph how he was attacked one evening in 1979, when she was eight years old:

“I was in the kitchen and my mother was sitting on the edge of the table and she just froze when the gunshots went off… I didn’t know what they were until my father came in on all fours crawling, with blood coming from his head… The terrorists thought he was on police duty and he would be coming home at 12 o’clock, but then they realised he was in the house. So when he went out to close the animals in for the night about 9.30 p.m. or so they opened up. That meant they were further away than they would have been if he had come in the car.”

She remembers her father describing how he “danced about” to try to avoid the bullets until he was hit in the head. He recovered from his injuries, but the family had to move.

Arlene joined the Girl Guides, which meant a lot to her, and went to the Collegiate Grammar School for Girls in Enniskillen. When she was 16, the IRA bombed her school bus in an attempt to kill the driver, who was a part-time member of the Ulster Defence Regiment:

“I was actually sitting beside a friend’s sister and I was in the inside and she was in the aisle. She was very badly injured. You then had to deal with the fact that this had happened and it could have been me, because we used to fight about who sat next to the window.”

She won a place to read law at Queen’s University Belfast, where she also chaired the Ulster Unionist Association, after which she practised as a solicitor, married Brian Foster, whose uncle was a prominent Ulster Unionist politician, and in 2003 was elected as an Ulster Unionist member for Fermanagh and South Tyrone to the Northern Ireland Assembly.

But she was a vehement opponent of the Good Friday Agreement, passed five years earlier, in the negotiation of which Trimble, the Ulster Unionist leader, had been centrally involved, and for which, with John Hume, he received the Nobel Peace Prize.

Foster explains that she was not against the idea of having an agreement, but was against this particular agreement “in terms of prisoner releases, the non-accountability of ministers and the emasculation of the RUC”. Soon after her election to the Assembly, she defected, along with Jeffrey Donaldson, from the Ulster Unionists to the DUP.

Her new party, which was in the ascendant, made her welcome, and from 2007 she served as a minister, and in early 2010 as Acting First Minister while Robinson was under a cloud. Her local paper in Enniskillen, The Impartial Reporterproudly reported that she was “the first woman and the youngest person to hold the top post in any United Kingdom devolved administration”.

At the end of 2015, Robinson retired and Foster succeeded him as DUP leader and First Minister. In May 2016, the DUP prospered in the Assembly elections. It also backed Brexit, so the following month found itself on the winning side in the EU referendum.

And in October 2016, as Gary Gibbon of Channel 4 News reported under the headline “Tories look to increase majority with DUP deal”, the DUP held a “champagne reception” at the Conservative Party Conference in Birmingham, attended by Gavin Williamson, the Conservative Chief Whip. Foster and Nigel Dodds, the leader of the party’s eight Westminster MPs, also paid a call during the conference on Theresa May.

It seemed Foster was modernising the DUP, turning it away from its sternly puritanical roots. Under Paisley, the idea of holding a reception at which alcohol – “the devil’s buttermilk” – was served would have been unthinkable.

But the new First Minister’s relations with Sinn Fein were less cordial. In December 2016, the “cash for ash” affair erupted, with Foster accused of having failed, after she introduced the Renewable Heat Incentive scheme, to act on warnings that costs were spiralling out of control.

In protest at her refusal to step aside while these allegations were investigated, the Sinn Fein leader Martin McGuinness resigned as Deputy First Minister, precipitating the collapse of the devolved government.

In March 2017, fresh Assembly elections had to be held, and the DUP emerged with only one seat more than Sinn Fein: a humiliating reversal of fortune for Foster. But this disaster was soon overtaken by the snap Westminster election called by Theresa May.

While the Conservatives lost their majority, the DUP gained two more seats, giving it a block of ten MPs and a very strong bargaining position. Williamson was at the heart of the negotiations with the DUP which secured the continuation of the Conservative government.

So there is unprecedented interest in what the DUP under Foster’s leadership is really like – a difficult question to answer, when one contemplates how many Ulster politicians have started by building a reputation for total intransigence, before spending the accumulated credit on doing a deal with their opponents.

Concentrating, as some in London are inclined to do, on the DUP’s social conservatism – its opposition to abortion and gay marriage – does not get one very far in working out its attitudes to constitutional questions.

But Foster’s record so far suggests she is happier holding to Unionist verities than overcoming her profound and understandable reservations, instilled in childhood, about cooperating with Sinn Fein.

She can react badly to criticism. Faced with opposition, she is more likely to dig her heels in, and try to bully her way through, than to charm or outmanoeuvre her adversaries.

At the DUP’s recent one-day conference, she was loudly cheered as she ruled out accepting any sort of special EU status for Northern Ireland, different from the rest of the UK.

Brexit has not made the DUP more British, but it has brought out that party’s pre-existing Britishness. As Foster said in her conference speech, in a deliberate retort to the claim during the Conservative Party Conference by the Sinn Fein leader Michelle O’Neill that “the North is not British”:

“Regardless of some of the propaganda the truth is the Union is secure and no matter how many times we are told that ‘the North isn’t British’, Northern Ireland is British and British it will remain.”

Paul Goodman and Henry Newman have warned in recent days on this site about the potential for the border issue to lead to disaster in the Brexit negotiations.

The Good Friday Agreement relied on a degree of ambiguity: on the ability of the different parties to interpret the settlement in different ways, focussing more on what they had gained than what they had lost.

There is a danger that Brexit, by bringing the national question into sharp relief, will dissipate that ambiguity. Thoughtful Unionists worry that one consequence could be to increase the generally static levels of support in Northern Ireland for a united Ireland.

The Stormont government remains in a state of collapse and the public inquiry into “cash for ash” has yet to report. Foster has shown resilience under pressure, which is indispensable, but has not yet demonstrated that she knows how to make the strange machinery imposed by liberal outsiders on Northern Ireland work.

For while any Unionist leader has for much of the time to reassure his or her followers by standing firm, he or she must also have the ability to see the right moment to disarm an opponent by making a concession. And whether Foster possesses that latter ability – so easy to commend from the safety of one’s armchair – is by no means clear.