He is one of the few lawmakers left in Haiti, a close ally of the assassinated president who has kept his seat while the country’s democratic institutions have been whittled away. As one of only 10 remaining members in all of Haiti’s Parliament, Rony Célestin, a swaggering figure who styles himself as a self-made multimillionaire, belongs to a tiny circle of leaders with the legal authority to steer the nation out of crisis now that the president is dead. But to many Haitians, Mr. Célestin is also a symbol of one of their biggest grievances: a ruling class that enriches itself while so many go hungry. In recent months, as the country erupted in protest over abuse of power by the political elite, Mr. Célestin has been parrying accusations of corruption from Haitian activists over his purchase of a mansion almost 2,000 miles away in Canada. The sprawling $3.4 million villa, with its sweeping driveway, home cinema, wine cellar and swimming pool overlooking a lake, was among the most expensive homes ever sold in one of Quebec’s most affluent neighborhoods, and the purchase set off a corruption investigation into Mr. Célestin by officials in Haiti.