MILAN — He may potentially be the most powerful man in Italy, yet few people know who he is. Foreign ambassadors seek him out, even though he holds no public office. He claims to be but a simple member of a political movement, volunteering free technical assistance, but critics say he and his small Milan company control the votes, the candidates and the policies of the country’s leading party.

As Italy faces critical national elections on Sunday, the media-shy internet entrepreneur, Davide Casaleggio, is the Wizard of Oz-like figure behind the tightly drawn curtain of the country’s front-running Five Star Movement as it approaches real political power.

Since its birth less than a decade ago, the party has confounded Europe’s establishment and promoted itself as an internet-based insurgent party that could change Italy’s politics. It has gained support, especially among young voters, by presenting itself as a truly transparent, egalitarian political movement that votes on its candidates — as well as positions — on the web.

Yet on the way to becoming a forum for unfiltered democracy, Five Star has grown into Italy’s most opaque political party, and Mr. Casaleggio its most enigmatic leader in an era when the web is a mire of hacking, fake news and hidden interests.