He is rich, handsome, openly gay, a former Goldman Sachs trader and shipowner to boot. Four weeks ago few in Athens had heard of Stefanos Kasselakis. But as the Greek American entrepreneur has shown, a month is a long time in politics. Since unexpectedly announcing his candidacy in the race to lead the leftwing Syriza party, Greece’s main opposition, the 35-year-old outsider has lit up the country’s staid political scene in ways few could have imagined. What might once have seemed outlandish – a US businessman vying for the leadership of the radical force – suddenly seems very real. Before...