As the United States settled into the second year of Donald Trump’s presidency in 2018, one figure was a constant if discreet presence. Robert Mueller, the man appointed to lead the investigation into Russian interference in the 2016 election, has emerged as a key figure in US politics, and could well be the man who determines the legacy of the Trump presidency.

In many ways, Mueller is the anti-Trump. While the US president trades in bombast and insults, Robert Mueller is notable for his silence, quietly going about his work at an office a few blocks away from the White House and the Trump Hotel on Pennsylvania Avenue.

Where Trump manipulates and courts the media, publicly airing his thoughts and grievances regularly on Twitter, Mueller has maintained a steely silence, declining to speak to the media and running a remarkably leak-free investigation.

Various documents and indictments have given a drip feed of information which have shed a light on the direction the Mueller investigation may be going

At 74, Mueller is just two years older than Trump. Both men were born into affluence and attended elite private schools, but the similarities end there.

Mueller studied at Princeton, graduating with a degree in politics in 1966. In contrast to Trump, who avoided the draft because of bone spurs in his ankles, Mueller joined the US marines and served in Vietnam, earning several military decorations. On return to the US he trained as a lawyer, working in private practice before serving in several US attorney offices.

By the late 1980s he had joined the department of justice, where he held senior positions. In 2001, George W Bush appointed him as the director of the FBI, after he had served as deputy attorney general. He was replaced by James Comey in 2013.

He returned to public prominence in May 2017, when deputy attorney general Rod Rosenstein appointed him as a special counsel to investigate “any links and/or co-ordination between the Russian government and individuals associated with the campaign of President Donald Trump, and any matters that arose or may arise directly from the investigation”.

The decision to appoint a special counsel was made in the aftermath of the firing of Comey by Trump. Rosenstein had the authority to choose Mueller because then attorney general Jeff Sessions had recused himself from the Russia investigation.

Since the establishment of the special counsel investigation, Mueller has appointed a team of about 15 lawyers who have been working from the office of the special counsel in the department of justice. It’s a formidable team, which includes Michael Dreeben, a top criminal law expert in the federal government, and Andrew Weissmann who previously worked on the Enron case.

Unusually for Washington, very little information has emerged from the investigation.

But the publicly available documents and various indictments filed by the prosecutors have given a drip feed of information which have shed a light on the direction the investigation may be going.

The special counsel team has been busy.

Within months of his appointment, Mueller had ordered a raid on the homes of Trump’s former campaign manager Paul Manafort and his long-time associate Rick Gates, with both men charged with money laundering and other charges in October 2017. Over the past year, the focus on current and former associates of the Trump team has continued.

In February, Mueller charged 13 Russian individuals and three Russian companies, including the St Petersburg-based Internet Research Agency, with conspiracy to tamper with the 2016 election. He would indict 12 more Russians four months later, charging them with breaking into the Democratic National Committee’s server during the election.

In April, FBI agents raided the home, office and hotel room of Trump’s long-term lawyer Michael Cohen in New York, on the instruction of the special counsel.

The move sparked a cascade of events that saw the man who once said he would “take a bullet” for Trump essentially defect and co-operate with the special counsel. Ultimately his decision to plead guilty failed to save him from a jail term – he was convicted to three years in prison on December 12th. However, he is not due to report to prison until March, sparking speculation that he could continue to co-operate with Mueller’s team and disclose further information in the coming months.

Much of Mueller’s public legal activity has focused on Manafort. He was Trump’s campaign manager in 2016 around the time when Russian players are believed to have hacked the DNCserver.

Manafort, a well-known lobbyist who undertook lucrative work on behalf of pro-Russian figures in Ukraine, was found guilty on eight charges of fraud by a court in Virginia in August. A month later he pleaded guilty to charges brought by Mueller, including conspiracy against the United States, entering a co-operation agreement.

But in an unexpected development, Mueller’s team in December announced that Manafort had in fact breached the terms of the agreement, amid reports that his lawyers had been briefing Trump’s legal team about his conversations with prosecutors after his plea deal. The unusual move prompted speculation that Manafort was seeking a presidential pardon.

Adam Schiff said Trump ‘may be the first president in quite some time to face the real prospect of jail time’ – though any indictment would take place only after he left office

In total, 33 people have been indicted by the special counsel, though most of those are Russians. Seven of those have pleaded guilty, including five from Trump’s inner circle.

As 2019 begins, there are signs that the Russian investigation may be close to conclusion. Unsurprisingly, activity related to the Mueller probe quietened down around the midterm elections, as the department of justice protected itself from any allegations that it could seek to influence the election.

But within weeks of the November midterms, Trump provided written answers to questions posed by the special counsel. Worryingly for Trump, there have been reports that John Kelly, the chief of staff he fired in early December, was among those interviewed by prosecutors. This could signal that the Mueller team is focusing on obstruction of justice by the president rather than simply any potential collusion between the Trump team and Russia during the election campaign, as Kelly was appointed chief of staff only in August 2017.

Throughout December, Trump has ramped up his attacks on the special counsel’s investigation, which he has repeatedly denounced as a “witch hunt.” “No collusion” has become a rallying call for Trump, and there are signs that his repeated characterisation of the Mueller probe as politically motivated and unjust is resonating with his supporters.

A December NPR/PBS poll found that seven out of 10 Republican voters agree with the president.

As the new Congress prepares to begin its two-year term in January, Democrats will now control the House of Representatives, a significant shift in power in Washington. Any move to impeach Trump after Mueller presents his report would begin in the House – though it would need a two-thirds majority in the Senate, which will remain in Republican hands for the next two years.

Democrats have so far refrained from threatening impeachment. During the midterm election campaign, House minority leader Nancy Pelosi instead encouraged her party to focus on bread-and-butter issues such as healthcare.

But in recent weeks senior Democratic figures have mentioned the I-word. Democrat Adam Schiff went as far as to say that Trump “may be the first president in quite some time to face the real prospect of jail time” – though any indictment would take place only after he left office.

Despite increasingly tough talk, the lessons of the Clinton impeachment 20 years ago still weigh heavily. Ultimately, Republicans were punished by voters for pursuing an impeachment trial the public did not want.

While the Mueller report may be the defining issue for Trump’s presidency in 2019, how Democrats respond to its findings will determine its impact on Trump.