Mohammad Javad Zarif, Iran's foreign minister, could be the key to solving Tehran's nuclear standoff with the West, having worked to resolve various crises in the Islamic republic since the 80s.
The 53-year-old diplomat's latest challenge is overseeing Tehran's team of negotiators in meetings with world powers at a new round of talks in Geneva on Thursday, as Iran seeks to lift crippling sanctions against its economy.
Zarif is fluent in English and is the only minister to have official accounts on Facebook and Twitter, both banned in Iran.
An international law PhD holder from University of Denver, he is popular with intellectuals and the youth, with nearly 500,000 followers on Facebook.
In charge of nuclear talks with the world powers, Zarif is a veteran loyalist of the Islamic revolution that toppled Iran's US-backed monarchy in 1979. He believes in the country's pursuit of nuclear power, which the West suspects is aimed at developing atomic weapons, despite Iranian denials.
But his moderate views on recent nuclear negotiations and foreign policy have clashed with ultraconservative factions in the regime.
An advocate of public diplomacy, he has years of experience in multilateral negotiations after serving as a diplomat to the United Nations for more than 20 years. He is also well versed in American culture and knows how to negotiate with the "Great Satan."
Raised in a religious family in Tehran, he said in his best-selling biography "Mr Ambassador" that he did not listen to music until he was 15. He also revealed that his wife -- then a devout revolutionary -- did not allow him to buy a television for nearly 10 years when they lived in the United States in the 80s.
But his wife became a follower of a moderate religious figure whose teachings changed "her into a quiet person filled with patience and tolerance".
"The new version had the greatest influence on our family", said Zarif, who has two grown children.
His involvement with politics goes back to his teenage years, when he participated in secret meetings in the run-up to the revolution.
At 16, his parents sent Zarif to California after he was threatened with arrest by the Shah's regime. There, he joined the Islamic Student Association and met many friends who later became political figures in Iran.
Among them was the brother of Akbar Hashemi Rafsanjani, president between 1989 and 1997.
Zarif also met, but did not get along with, Saeed Emami, a hardliner who later became a deputy minister of intelligence. Emami committed suicide in prison after being arrested in 1999 for the assassination of political dissidents.
"The New York Clique"
Following the seizure of the US embassy in Tehran by Islamist students and the subsequent severance of ties with Washington, Zarif was sent to shut down Iran's consulate in San Francisco.
He then studied international relations, to better serve the Islamic republic, he said, and wrote a dissertation on "sanctions in international law."
In the late 1980s, he joined the Iranian delegation at the UN and was ambassador from 2002 to 2007, during the presidency of reformist Mohammad Khatami.
But hardline president Mahmoud Ahmadinejad sacked him, and his stint in the US earned him the hostility of the ultraconservative camps, who denounced the "New York Clique" and Zarif's leading role in it.
He was tipped for the post of foreign minister by President Hassan Rouhani, a moderate who won a surprise victory against conservative candidates in presidential elections in June.
Zarif won a vote of confidence -- 232 in favour out of 280 votes cast -- of the conservative-dominated parliament for the post, after showing off his oratorical skills in a speech urging moderation and with references to the Muslim holy book, the Koran.
He also has a record of taking part in key negotiations.
Zarif was a member of Iranian delegations which negotiated a ceasefire with Iraq, secured the release of American hostages in Lebanon and managed to convince Tehran to assist the US against the Taliban and Al-Qaeda in Afghanistan after the September 11 attacks.
In 2003, he accompanied Rouhani to negotiations with the three European powers -- Britain, France and Germany -- in which Iran agreed to halt sensitive enrichment activities and strengthen international supervision of its nuclear sites, an agreement later terminated by Ahmadinejad.