Washington (CNN) When Gordon Sondland arrived in Brussels as the US ambassador to the European Union in July 2018, it didn't take long before people were reminded of his American boss.

The hotel executive struck many of his European counterparts as something of a mini- Donald Trump . He started renovating the US embassy. He freely gave interviews and was a fixture on Twitter. He threw lavish parties and boasted of his growing relationship with Trump.

In the corridors of the European Parliament, Sondland worked to build personal relationships with heads of state and diplomats. Though he knew it was a breach of protocol, Sondland's habit of calling officials by their first name was an attempt to build a personal rapport, but also struck some as off-putting in the rule-bound capital of the European Union. So did his penchant for speaking bluntly.

One veteran diplomat tells CNN that Sondland would ridicule the EU for being obsessed with regulations. "At the same time, he was open and candid -- just not a believer in the traditional values that have kept Europe and the US so close in the last 70 years."

In discussing trade, Sondland would complain that the EU was "ripping off" the US, said Jeremy Shapiro, a research director at the European Council on Foreign Relations, who interacted with Sondland. Shapiro says that Sondland would also threaten his European counterparts with a "Chinese-style stand-off" if they didn't cooperate on trade.

"When he speaks he's forceful, blunt and un-diplomatic and it aggravates people," said Shapiro.

A key player

JUST WATCHED Sondland's role in Trump's impeachment inquiry explained Replay More Videos ... MUST WATCH Sondland's role in Trump's impeachment inquiry explained 02:48

Sondland is now a key player in an impeachment inquiry examining whether the President tried to use US military aid as leverage in his effort to get a foreign country to investigate a political foe.

On Thursday, Sondland, who remains in his post as ambassador, will testify behind closed doors to the House Intelligence, Foreign Affairs and Oversight committees that are leading the Democratic impeachment inquiry into Trump and Ukraine. Their investigation was spurred by a whistleblower complaint alleging Trump sought help from Ukraine to investigate former Vice President Joe Biden and his son Hunter's position on the board of a Ukrainian natural gas company, Burisma Holdings.

Sondland is a crucial witness for Democrats' impeachment case. Lawmakers have already interviewed multiple current and former State Department officials, and Sondland has popped up in their testimony in several instances, including text messages turned over to Congress in which Sondland and a top US diplomat in Ukraine debate whether $400 million in military and security aid was being withheld in connection with Ukraine opening an investigation.

"I think it's crazy to withhold security assistance for help with a political campaign," acting US ambassador to Ukraine Bill Taylor texted to Sondland on September 9.

Taylor is expected to testify before the committees on Tuesday.

A self-made hotelier from Portland, Oregon, Sondland was for years the Republican Party's go-to bundler in the Pacific Northwest. After donating to the presidential campaign of Mitt Romney in 2012, Sondland had initially supported Jeb Bush and then Marco Rubio in their 2016 presidential campaigns.

The irony of his current predicament is that he was not welcome in Trump World and had to force his way in, according to multiple former White House officials.

According to two Trump campaign advisers, Sondland angered Trump when he denied his campaign the use of his hotels as fundraiser venues after Trump's negative remarks about the family of a slain Muslim-American soldier in the summer of 2016. One of Sondland's business partners is Muslim.

But after Trump won, Sondland found himself on the outside looking in. "Gordon was desperate to win the approval of Trump," said one of his long-time Republican friends who worked in the White House. To earn Trump's favor, Sondland ended up donating $1 million to the inauguration.

Sondland's change of heart did not surprise people who know him. David Nierenberg, a wealthy investor and friend of Sondland, told CNN that Sondland always aspired to connect himself to people of power.

"People like me collect books and wine and Gordon collects relationships with people in power," said Nierenberg, adding that a glimpse of Sondland's personal office offers a telling window into his ambition. Lining the room are photos of Sondland alongside well-known politicians, including Romney, and both Presidents Bush. What's striking, says Nierenberg, is that pictures of Sondland with politicians outnumber those of his own family. "You can see his ambition in the photographs."

Remarkably unremarkable

After his inauguration donation, Sondland's name was suddenly on a list that former White House adviser responsible for personnel, Johnny DeStefano, regularly put in front of the President, says a person with knowledge. Still, it took Trump months to come around to the idea of nominating Sondland.

One Senate aide remembers that given all the other Trump nominees deemed unfit by many Democrats, Sonland seemed harmless. "He blended in the background," the aide said.

"What is maybe most remarkable in retrospect is how unremarkable his nomination seemed at the time," said another Congressional source familiar with Sondland's confirmation. "He was very obviously a donor pick and not someone with much prior foreign policy experience, but other than that, our records and our staff's recollections of it are that there wasn't much else that stood out during his confirmation process. I definitely don't think at that time that anyone guessed he would have ended up in the middle of a fiasco like this."

Once confirmed, Sondland was viewed as a problem by the national security teams at the White House and at the State Department, two administration officials told CNN. Sondland did not feel bound by Secretary of State Mike Pompeo or national security adviser John Bolton. He could call the President whenever he wanted, and did, they said.

Bolton's spokesperson declined to comment. A State Department spokesperson also declined to comment.

President Donald Trump is joined by Gordon Sondland, the U.S. ambassador to the European Union as he arrives at Melsbroek Air Base, in Brussels, Belgium in July 2018

"Once he was an ambassador he was going to put himself in the middle of things, that's who he is," says a Republican political consultant.

Sondland would reach out to Bolton at times to try to set up meetings, but Bolton would often refuse to meet him, one of the administration officials explained.

That same official told CNN that Sondland would often confuse other countries that dealt with him, because he blurred the lines of who was in charge of administration policy. Sometimes the White House or State Department would ignore his initiatives because they were not always consistent with policy, the official said.

Ukraine is not usually in the portfolio of the EU embassy, but Sondland may claim it was always in his briefing book, according to a source familiar with what he is likely to testify.

In her testimony to Congress this week , Fiona Hill, Trump's former top Russia adviser, said that she regarded Sondland as a security risk, a source familiar with her testimony tells CNN. Hill told Congress that Sondland frequently used his personal cell phone to conduct official diplomatic business, and that she worried his lack of experience could be exploited by foreign governments.

What will he say?

Sondland is still expecting to show up on Thursday and speak to impeachment investigators, according to a person familiar with the EU ambassador's preparations. Sondland's legal team has publicly pointed to the State Department regarding any document requests, saying State would be the entity to turn any discussions of official business over to the Hill.

Sondland hasn't heard from the White House or State Department with requests to block all or parts of his testimony Thursday, the person said. Last week, the State Department stopped Sondland from speaking to Hill investigators, prompting them to subpoena him. Hill's legal team had refused to honor a request from the White House to consider some of her communications privileged.

Sondland, the source said, is pushing back on the idea that the White House's national security advisers expressed alarm about a possible quid pro quo with Ukraine, saying that Hill and Bolton never made it known to Sondland that they were concerned.

Even emails between Sondland and Hill, Trump's former Russia specialist, were "cordial" and "collegial," with the ambassador keeping her updated on his activities in Ukraine, the person said.

Sondland contends that he never understood that the President sought an investigation of Joe Biden, according to the source, even when Trump's personal lawyer Rudy Giuliani was pushing for a Ukrainian investigation of the natural gas company Burisma and into foreign interference in the 2016 election. Sondland contends he did not understand that Trump sought political help from Ukraine at least until after the July 25 call transcript was released, where Trump had brought up Biden, according to the source.

The source also contends Sondland's interactions with Giuliani were far less than others perceived, though necessary to achieve progress with Ukraine.

"If you wanted to get this meeting with (Ukrainian President Volodymyr) Zelensky, then you could either abandon that or you could work through Rudy (Giuliani)," the person said, again describing what Sondland may say on Thursday. "They came to understand that what Rudy wanted was a public statement by Zelensky to look at Burisma and the server in 2016."

Some of Sondland's friends worry about his potential exposure. "I hope that he did not cross any lines that he might now regret," said Jordan Schnitzer a real estate developer in Portland, Oregon, and a longtime friend of Sondland.

Sondland's friend David Nierenberg told CNN that he had emailed Sondland as the Ukraine scandal was breaking, reminding him that Richard Nixon did not lose the presidency because of the break-in at Democratic National Committee, but rather because of two years of lies and cover-ups. "I concluded my message, therefore, by saying whatever you do, tell the truth, if there's anything that you regret doing or not doing on reflection that admitted, freely, apologize for it and make things right because you have the rest of your life ahead of you."

"Thank you," Sondland replied. "You've always given me good advice."

The story has been updated to clarify what Sondland understood the President sought from Ukraine before he saw the transcript of the call with Zelensky.