They seek him here, they seek him there, those snappers seek him everywhere but the billionaire pimpernel Paddy McKillen rem­ains a man of mystery.

His constitutional challenge to Nama's decision to suck up his €2.1bn loans owed to the various banks has put him in direct confrontation with the state. If he becomes 'The Man Who Broke The Toxic Bank', he will be adored and reviled in equal measure. If his case is successful, many fear it could topple the country into the arms of the IMF as 'the markets' have a panic attack. But if McKillen fails, will he throw in the towel in frustration and lose faith in investing in Ireland, just at a time when the country needs all the tough entrepreneurs it can get?

The stakes couldn't be higher. The businessman ("I'm not a developer") is employing three senior counsel and nine expert witnesses, including the Nobel prizewinning economist Joseph Stieglitz to back his High Court case that Nama's decision to take over his loans contaminates his ability to cut a deal by toxic association.

In court, there are references to the 62 properties he owns in this country, the chain of five-star hotels in Britain, property in France and the retail and offices in the United States, all of which, he argues, are making enough to support his loans.

But where is that 55-year-old with the Belfast burr, whose only published picture is of a slim, kindly-looking man in a monkey suit and a bow tie on a formal night out years ago? Typically frivolous, the obsession with getting the first photo has become as big as the story itself for the assembled media. So far, McKillen has retained his anonymity, but his business presence is everywhere.

He took big risks back in 1995 to co-develop Dublin's Jervis Centre. McKillen is said to have worked night and day, arriving on site once at 3am to watch a critical cement-pouring. The 'Jervo' became the template for similar centres – but more influentially for Irish society, it opened the floodgates to the damburst of British retail chains that now dominate our shopping landscape.

It all started humbly. At 16, McKillen left school and home in Andersonstown, to work in DC Exhausts, a firm with family connections. By the '90s, he'd sold it at a profit.

There is no sentiment in McKillen's business view: he is a co-owner of the Treasury building where Nama is headquartered and he also has a stake in the Boston skyscraper where Anglo Irish Bank's US headquarters struggles on.

McKillen is described as pleasant, a family man with three sons, and a non-drinker, who enjoys rugby. His wife is Maura McMenamin, a woman admired in her 20s as Ireland's top model. He owns a house in Foxrock, Dublin, but is said to spend fewer than 20 days a year there. The family criss-cross between LA and spend a lot of time in London.

This case says a lot about his character. The soft- spoken demeanour belies a steely core. Nobody messes with his right to run his businesses profitably. Nama can be his tenant, but not his banker.