Part Italian, part Swedish, Mr. de Mistura has worked with the United Nations for more than 40 years, but he is more widely known for his dapper style than for any diplomatic coups. Syria is by far the toughest assignment of his career — indeed, two of the organization’s most seasoned diplomats, Lakhdar Brahimi and Kofi Annan, tried to do the job and gave up — and critics have wondered aloud whether Mr. de Mistura is up to the task.

He served as a United Nations envoy in Afghanistan and Iraq, and before that in Lebanon, where a former minister recalled, with some scorn, that he spent many hours sunbathing at a private club in the hills above Beirut. Those who know him say he has a taste for fine suits and can sometimes speak too soon and too much, just as they point to his diplomatic missteps and hyperbole.

They cite, for instance, a news conference in October, when he raised the specter of Srebrenica, where thousands of Muslims were massacred in 1995 during the Balkans war, in warning that the Syrian border town of Kobani could fall to the Islamic State. In February, he was photographed at a party in Damascus, the Syrian capital, celebrating the anniversary of the Iranian revolution just as Syrian forces, aided by Iran, were pummeling rebel-held suburbs of Damascus; critics seized on that as evidence of his coziness with the government.

Mouin Rabbani, who served briefly as the head of Mr. de Mistura’s political affairs unit and has since emerged as one of his most outspoken critics, said Mr. de Mistura did not have the background necessary for the job. “This isn’t someone well known for his political vision or political imagination, and his closest confidants lack the requisite knowledge and experience,” Mr. Rabbani said.

As a deputy foreign minister in the Italian government, Mr. de Mistura was tasked in 2012 with freeing two Italian marines detained in India for shooting at Indian fishermen. He made 19 trips to India, to little effect. One marine was allowed to return to Italy for medical reasons; the other remains in India.

He said he initially turned down the Syria job when the United Nations secretary general approached him last August, only to change his mind the next day, after a sleepless, guilt-ridden night.

Mr. de Mistura compared his role in Syria to that of a doctor faced with a terminally ill patient. His goal in brokering a freeze in the fighting, he said, was to alleviate suffering. He settled on Aleppo as the location for its “fame,” he said, a decision that some questioned, considering that Aleppo was far trickier than the many other lesser-known towns where activists had negotiated temporary local cease-fires.