The star witness in Donald Trump's indictment is a man who once vowed to "take a bullet" for the former commander-in-chief.

For a decade, Michael Cohen served as Mr Trump's lawyer and backroom fixer, working, in his own words, to cover up his ex-boss's "dirty deeds".

But since becoming mired in legal trouble of his own, Mr Trump's one-time henchman has transformed from archdefender to archnemesis.

It was Cohen who facilitated, and was later jailed for, the hush money payment that could now lead to Mr Trump becoming the first former US president to be indicted.

'Dirty deeds'

As he stood on the steps of a New York courthouse earlier this month, Cohen, 56, signalled he was primed to play a central role in Mr Trump's potential arrest.

“This is all about accountability. Donald Trump needs to be held accountable for his dirty deeds," he told reporters, before testifying to the grand jury which will decide the ex-president's fate.

The case against Mr Trump centres on a $130,000 payment to silence Stormy Daniels, a porn star, over an alleged affair.

Mr Trump has always denied the claim and denounced the investigation as a politically motivated "witch hunt".

Cohen pleaded guilty in 2018 to campaign finance violations over the hush money and other charges, including tax evasion and lying to Congress about a Trump business deal.

He served more than a year in jail and testified in court that Mr Trump "directed him" to commit a crime by making the hush payment with the express intention of influencing the election.

Prosecutors are now relying on Cohen's evidence to build a case that Mr Trump should face charges over the payment.

But critics say his conviction for lying, in particular, makes him a problematic witness.

'The guy who stops the leaks'

Described as a former "mega fan" of Mr Trump, 76, Cohen landed a job with the real estate mogul and reality TV star in 2006 after impressing him with his ability to anticipate his needs.

Cohen's career route to becoming the personal lawyer for a US president was far from orthodox.

A native of Long Island, New York, Cohen attended Western Michigan University’s Cooley Law School, which has since earned a reputation as one of the worst in America.

He worked for a time as a personal injury lawyer, but built his wealth buying and selling taxi medallions in New York's murky taxi badge industry.

Later, as the personal lawyer to one of New York's biggest real estate magnates, he was handling business deals worth considerably more.

Cohen believed his remit extended beyond official business. "I'm the guy who stops the leaks. I'm the guy who protects the president and the family. I'm the guy who would take a bullet for the president," he told Vanity Fair during Mr Trump's first year in office.

However, Cohen was said to be hurt that Mr Trump did not offer him a role in the White House.

Turning on Trump

Pressure mounted on Cohen when the FBI raided his home and offices and seized hundreds of items in relation to investigations into Mr Trump.

A row over who would foot the bill for his legal fees and the looming threat of a lengthy stretch in prison then paved the way for Cohen to later turn on his former boss.

He has since taken a scorched-earth approach, publishing a book titled, Revenge: How Donald Trump Weaponized the US Department of Justice Against His Critics.

In an interview with the Telegraph last year, Cohen hinted at what is driving his continued animus toward his former boss.

"Donald has never accepted responsibility for any of his improper actions and never will," he said.