Washington (CNN) Ambassador Bill Taylor came out of retirement to lead the US embassy in Kiev -- a decision he did not take lightly. Now the longtime diplomat is a central figure in the House impeachment inquiry into President Donald Trump and will become one of the first witnesses to testify publicly as part of the probe.

Taylor is expected to appear on Wednesday in a joint hearing with George Kent , a deputy assistant secretary of state in the Bureau of European and Eurasian Affairs.

The testimony from the charge d'affaires in Kiev -- provided at a closed door hearing in late October -- has become a cornerstone in the Democrats' bid to prove Trump's alleged quid pro quo with Ukraine.

A dozen former State Department officials who spoke to CNN about Taylor described him as a person of high character and professionalism -- a "very quiet guy," in the words of retired Ambassador Ronald Neumann -- who is deeply respected in the diplomatic world and seen as more likely to put sound foreign policy before politics.

"He's somebody who's been willing to take hard jobs," Neumann told CNN.

Concerned about serving in a 'snake pit'

Taylor will testify in Wednesday's open hearing having already spoken in extensive detail about his concerns related to Ukraine.

According to a transcript of his closed-door testimony, Taylor told House impeachment investigators that Trump's personal attorney Rudy Giuliani was pressing Ukraine "to intervene in US domestic policy or politics" by launching investigations into the President's political rivals. It was his "clear understanding" US security aid to Ukraine wouldn't be released unless Ukrainian President Volodymyr Zelensky announced such investigations, Taylor testified last month. And he said that it was Giuliani, representing Trump's interests, who was behind what then-national security adviser John Bolton described as a "drug deal."

Taylor also testified to his concerns about taking the post in Kiev in the wake of former Ambassador Marie "Masha" Yovanovitch's unexpected removal in May.

"I was concerned that there was, I think I put it, a snake pit in Kiev and a snake pit here, and I was not sure that I could usefully serve in that context," Taylor said, according to the transcript.

Taylor had previously served as the US ambassador to Ukraine from 2006 to 2009, but at the time he was asked to take over at the embassy, he had retired from the Foreign Service and was serving as executive vice president at the US Institute of Peace.

"He was appalled, as we all were, when Marie Yovanovitch was removed prematurely," a retired US ambassador told CNN.

Taylor told House investigators he spoke with Secretary of State Mike Pompeo and State Department Counselor Ulrich Brechbuhl, where he raised concerns about a potential shift in US policy on Ukraine, telling Pompeo he would quit if it changed.

"And I told the Secretary: If that happens, I'll come home. You don't want me out there, because I'm not going to defend it, you know. I would say bad things about it. And you wouldn't want me out there doing that. So I'm going to come home on that," Taylor said.

Taylor was eventually persuaded to take the role, and he arrived in Kiev in June.

'Someone of extreme integrity'

Many of former officials told CNN they were unsurprised that he was chosen for the embassy job and said State Department officials who work on Ukraine were pleased with the choice.

"It certainly provided a morale boost to the embassy when he arrived," another former State Department official said, noting that the embassy had been "pretty battered" by the false allegations against Yovanovitch.

One former senior State Department official said it was "a little unusual" that Taylor was called out of retirement, noting that it requires decisions and special dispensations involving pensions and pay.

"It's not an easy thing to do," that official said.

After his testimony, Trump claimed, without evidence, that Taylor was a "Never Trumper" and the White House suggested that he and other diplomats were "unelected radical bureaucrats."

However, officials who spoke with CNN portrayed Taylor as a consummate professional -- "someone of extreme integrity and a very solid set of principles," in the words of retired Ambassador Nancy McEldowney.

Others noted that Taylor did not need to return, but did so out of a sense of diplomatic obligation.

A source familiar with his career said Taylor took the job to be a good soldier. The former senior State Department official described Taylor as having "a remarkable legacy of service."

"He is one of the real superstars of US foreign policy," Tara Sonenshine, a former Undersecretary of State for Public Diplomacy and Public Affairs, told CNN.

Taylor is a graduate of the US Military Academy at West Point and a Vietnam War veteran. Three veterans who served with him defended him as a "man of honor," "public servant" and "role model" in interviews with CNN.

In addition to his diplomatic service in Ukraine, he oversaw assistance and support to Egypt, Tunisia, Libya and Syria at the State Department during the Arab Spring, "served in Jerusalem as the U.S. Government's representative to the Mideast Quartet," "oversaw reconstruction in Iraq from 2004 to 2005, and served in Kabul as coordinator of international and U.S. assistance to Afghanistan from 2002 to 2003," according to his State Department biography.

This story has been updated ahead of Taylor's public testimony on November 13.