Rex Tillerson - the wild card diplomat
US Secretary of State Rex Tillerson has kept a low profile during his first few months in the role.
His reluctance to engage with the media in the free-wheeling way of his predecessor has sparked criticism, according to the BBC's defence and diplomatic correspondent, Jonathan Marcus.
This is partly due to his inexperience in the field, and partly due the Trump administration not clearly fleshing out its key areas of approach, our correspondent adds.
The former oil executive is new to the diplomatic world.
The 64-year-old Texan spent his entire career working for the world's most valuable, publicly-traded oil company, Exxon Mobil.
He worked his way up to becoming chief executive officer and was set to retire when President Donald Trump tapped him up for one of his government's biggest roles.
While critics raised concerns about his ability to trade in his corporate interest for a national one, his supporters said his background as a global dealmaker would bring fresh perspective to the nation's top diplomatic post.
However, his links to Russia were hotly debated in the press and among US lawmakers.
Mr Tillerson had worked in Russia, overseeing some major oil contracts, and he was known for having close ties with Russian President Vladimir Putin.
His links to the Kremlin, which awarded him the Order of Friendship in 2013, was the main subject of scrutiny at a Senate hearing before he was appointed.
During a heavy grilling, he admitted that the West had reason to be alarmed by Russian aggression but he refused under questioning to label Mr Putin a war criminal.
He was narrowly approved for the role, with only a handful of Democrats crossing party lines to back him.
On the eve of his first state trip to Moscow, he was notably critical of the country, suggesting the Kremlin was either incompetent or complicit after Syria was alleged to have used chemical weapons.
The Syrian government had previously agreed to surrender chemical weapons under the supervision of its Russian allies.
'I'm not a big media person'
Mr Tillerson also came under fire over his first state visit to Asia in March, when, instead of letting the state department press corps accompany him, he took only one reporter from conservative website Independent Journal Review (IJR).
In the IJR interview, Mr Tillerson said: "I'm not a big media press access person. I personally don't need it."
In the same interview, he said his wife persuaded him to take the job.
He recalled going to visit Mr Trump, thinking he was just sharing thoughts about the world, and was shocked when he was offered the role.
Friend of Russia?
Mr Tillerson's nomination followed revelations that US intelligence agencies believe Russia acted covertly to help Mr Trump defeat Hillary Clinton in the election, leaving some critics unnerved by his close relations with Moscow.
During his time at Exxon, Mr Tillerson has forged multibillion-dollar deals with Russia's state oil company, Rosneft, including an agreement to explore underground resources in Siberia that could be worth billions of dollars.
He is also known to be a friend of Igor Sechin, Rosneft's executive chairman who was formerly Mr Putin's deputy prime minister. Mr Sechin has been called Russia's second most powerful man.
Mr Tillerson has publicly spoken out against international sanctions placed on Russia over its annexation of Crimea.
In 2014, Exxon filed a report saying the US government and European Union's sanctions cost the company a maximum of $1bn (£790m) in damage to joint ventures.
Republican senators Marco Rubio and John McCain expressed serious concerns about Mr Tillerson's Russian connections but came round.
Arizona Senator John McCain said he was reassured by private conversations he had with the oil chief about his views on Russia.
"This wasn't an easy call, but I believe when there's doubt, the incoming president gets the benefit of the doubt," he said.
Mr Tillerson beat a long list of seasoned candidates in the running for the secretary of state post, including former Massachusetts Governor Mitt Romney; Senator Bob Corker, the chairman of the Senate Foreign Relations Committee; and former CIA chief David Petraeus.
Though Mr Tillerson appears to fit with Mr Trump's right-wing cabinet, he has stirred controversy among some social conservatives who condemn his more liberal policies.
As president of the Boy Scouts of America from 2010-2012, Mr Tillerson was part of the push to allow gay scouts and leaders into the organisation, although a ban on openly gay adult scout leaders remained in place until 2015.
Evangelical leader Tony Perkins, the president of the Family Research Council, slammed Mr Tillerson's nomination, noting Exxon's history of donating to family-planning organisation Planned Parenthood.
Mr Tillerson is also a former director of the United Negro College Fund, a US organisation that funds scholarships for black students and supports historically black colleges and universities.
Fossil fuel advocate
Exxon, which has about 75,000 employees around the world, has been accused of trying to cover up the risks of climate change and lying to the public.
The company has dealt with a series of state investigations into how much it knew about climate science, leaving many environmental activists concerned about his appointment.
Yet Mr Tillerson accepts climate change is real and has spoken of "catastrophic" consequences if it were left unchecked.
"For many years, Exxon Mobil has held the view that the risks of climate change are serious and do warrant action. We believe that addressing the risk of climate change is a global issue," he said last May.
Though he is so far one of the few members of Trump's cabinet to acknowledge the existence of man-made climate change, Mr Tillerson remains committed to the continued use of fossil fuels.
He is, however, open to the idea of a carbon tax to reduce emissions, a view likely to clash with those held by colleagues in government.