Gordon Sondland is a wealthy hotelier, a Republican mega-donor and President Donald Trump's hand-picked ambassador to the European Union — and the man who witnesses say directly connected Trump to the freeze on security aid to Ukraine and demands that it commit to investigating the 2016 U.S. election and former Vice President Joe Biden.

Sondland, set to testify publicly in the House impeachment inquiry on Wednesday, was one of the so-called three amigos who emerged as the point men on U.S. dealings with Ukraine following the ouster of Ukraine Ambassador Marie Yovanovitch in May.

He amended his initial testimony in the case this month to acknowledge that he'd told a top Ukrainian official in September that millions in security aid would be withheld "until Ukraine provided the public anti-corruption statement that we had been discussing." He will have more to explain Wednesday after additional disclosures by other witnesses.

Donor turned diplomat

Why Sondland is important?

The E.U. ambassador has been placed at several key events at the heart of the impeachment inquiry.

He testified that he attended an Oval Office meeting on May 23 with the other two "amigos" — then-special envoy to Ukraine Kurt Volker and Energy Secretary Rick Perry — and the president, where Trump vented about corruption in Ukraine and told the trio to talk to his personal lawyer, Rudy Giuliani.

He was involved in a July 10 meeting with Ukrainian officials, national security adviser John Bolton and aide Fiona Hill that Bolton ended abruptly after Sondland told the Ukrainian officials they needed to carry out investigations if they wanted an official White House visit, according to testimony from other participants.

Sondland spoke to Trump a day after the July 25 phone call with Ukraine President Volodymyr Zelenskiy, telling Trump in a cellphone conversation overheard by three other people that Zelenskiy had agreed to carry out investigations desired by Trump and Giuliani. According to a witness who testified about the call, State Department official David Holmes, Sondland then said that Trump "only cares about 'big stuff'" when it comes to Ukraine, like the "Biden investigation."

In his supplemental testimony, Sondland recalled telling a top aide to Zelenskiy on Sept. 1 "that resumption of U.S. aid would likely not occur until Ukraine provided the public anti-corruption statement that we had been discussing for many weeks. I also recall some question as to whether the public statement could come from the newly appointed Ukrainian prosecutor general, rather than from President Zelenskiy directly."

The top U.S. diplomat in Ukraine, Bill Taylor, texted Sondland after hearing about that conversation asking if the U.S. officials were "now saying that security assistance and WH meeting are conditioned on investigations?" Taylor said Sondland told Taylor to call him. "During that phone call, Ambassador Sondland told me that President Trump had told him that he wants President Zelenskiy to state publicly that Ukraine will investigate Burisma and alleged Ukrainian interference in the 2016 U.S. election," Taylor testified. "Ambassador Sondland said, 'everything' was dependent on such an announcement, including security assistance. He said that President Trump wanted President Zelenskiy 'in a public box' by making a public statement about ordering such investigations."

On Sept. 9, Taylor texted Sondland that "as I said on the phone, I think it's crazy to withhold security assistance for help with a political campaign." Sondland said he spoke to Trump after getting that text and asked him, "What do you want from Ukraine?" "The president responded: 'Nothing. There is no quid pro quo,'" he said in his initial testimony on Oct 17.

In his supplemental testimony, Sondland said that "I always believed that suspending aid to Ukraine was ill-advised, although I did not know (and still do not know) when, why, or by whom the aid was suspended."

Trump called Sondland "a really good man and great American" before he was first deposed in October, but told reporters "I hardly know the gentleman" after he supplemented his testimony this month.

Who else is testifying this week?

Following Sondland's testimony, which is set for 9 a.m. ET Wednesday, the committee will hear from Laura Cooper, the deputy assistant secretary of defense for Russian, Ukrainian and Eurasian Affairs and David Hale, the undersecretary of state for political affairs. Their testimony is expected to begin at 2:30 p.m. ET.

On Thursday, the committee will hear testimony from Fiona Hill, the former National Security Council senior director for Europe and Russia involved in the July 10 meeting, and Holmes, the official who overheard the July 26 conversation between Sondland and Trump. Their testimony is scheduled to begin at 9 a.m. ET.

Who's doing the questioning?

House Intelligence Committee Chairman Rep. Adam Schiff and ranking member Rep. Devin Nunes, both of California, will have 45 minutes each to question the witnesses at the beginning of each hearing. They can — and are expected to — delegate the bulk of that questioning time to their committee lawyers instead. Daniel Goldman, a former federal prosecutor, is the lawyer for the Democrats, and Steve Castor is the lawyer for the Republicans.

Schiff will determine if there is a need for an additional 90 minutes of questioning by the lawyers before moving on to lawmaker questions. There are 13 Democrats and nine Republicans on the panel.