Washington (CNN) Vice President Mike Pence's role in the events leading to the House of Representatives' impeachment inquiry is expected to come under scrutiny Thursday as a top aide testifies on Capitol Hill.

Williams was one of the nearly dozen officials listening on President Donald Trump's July 25 phone call with Ukrainian President Volodymyr Zelensky , according to two sources familiar with the matter.

Williams reports to Gen. Keith Kellogg, who directly advises Pence on national security. Williams is expected to remain in Pence's office until the spring of 2020, when her tenure is up.

She was concerned about what she heard on the call but there is no indication Williams raised her concerns to her superiors, according to one of the sources.

"We're also not familiar with her expressing any displeasure to a General Kellogg either," said Katie Waldman, Pence's press secretary.

Democrats say they have questions about her knowledge of how much Pence knew about efforts by Trump and those around him to push Ukraine to launch investigations into Joe Biden and his son, as well as 2016 election interference. There is no evidence of wrongdoing of either Biden in Ukraine.

Though Pence was not on that call, he has met with and held a call with Zelensky himself.

The President promised to release a transcript of that call, which has, so far, failed to materialize. Pence has largely stayed under the radar as the House has proceeded with closed-door interviews, but more details on his role and what he knew are expected to be revealed by Williams' testimony

Lawmakers will look to Williams to explain what Pence knew and when, a source familiar with Williams' thinking says she is able to do.

Williams' allies were surprised when she was called to testify. Her name had not been mentioned yet despite multiple efforts to nail down who was on the ill-fated July call, and many of her colleagues went to great lengths to keep her name out of it, according to multiple people aware of the situation.

One of the concerns for the non-political staffers who have been caught up in the Ukraine scandal was being forced to hire legal representation.

Justin Shur, Williams' attorney, told CNN in a statement Wednesday night that she would answer the committee's questions "if required to appear."

"Jennifer is a longtime dedicated State Department employee," Shur said. "If required to appear, she will answer the Committees' questions. We expect her testimony will largely reflect what is already in the public record."

So while Williams appeared before House investigators Thursday, she was not an eager witness.

Inside the Eisenhower Executive Office Building, Williams is seen as a total professional by her colleagues, who heaped praise on her when asked about her work efforts. Williams was detailed to Pence's office earlier this year, and played a role in Pence's recent negotiations for the ceasefire between Turkey and Syrian Kurdish forces, according to a White House official.

She is described by officials familiar with her standing as competent, but someone who would voice objections when she believed it was necessary.

A canceled trip

Pence's involvement in key events leading to the impeachment inquiry began as the administration compiled a delegation to attend Zelensky's inauguration this spring and extends through a September trip to Ukraine and call with the Ukrainian President.

According to the whistleblower's declassified complaint , Pence had been expected to lead the US delegation to Zelensky's inauguration on May 20, but was instructed by Trump to cancel the trip. Energy Secretary Rick Perry traveled to Ukraine in his stead.

"I learned from US officials that, on or around 14 May, the President instructed Vice President Pence to cancel his planned travel to Ukraine to attend President Zelensky's inauguration on 20 May; Secretary of Energy Rick Perry led the delegation instead. According to these officials, it was also 'made clear' to them that the President did not want to meet with Mr. Zelensky until he saw how Zelensky 'chose to act' in office," the whistleblower's report said

Testimony from Williams could reveal more details about why Pence did not go on the trip.

In his testimony before House committees, US Ambassador to the European Union Gordon Sondland said he "had heard" Pence was considering attending Zelensky's inauguration but did not know why he did not ultimately go. And former US Special Envoy to Ukraine Kurt Volker said he was not aware that Trump directed Pence not to go: "I assumed it was just a matter of scheduling."

Two of Pence's staffers were on the July 25 call between Trump and Zelensky, including Williams, as well as Gen. Keith Kellogg, the vice president's national security adviser, who has not yet been called to testify. Pence did not listen in, but was told about the call the day afterward, according to two sources familiar with the matter. A transcript of the call was put into Pence's daily briefing binder, according to three sources

"He may or may have not read it," one source said.

Last-minute journey to Poland

Williams' testimony could shed light on Pence's awareness of what Trump said on the call about Joe and Hunter Biden.

On September 1, Pence made a last-minute trip to Warsaw, Poland, in Trump's stead due to Hurricane Dorian. The vice president held a bilateral meeting with Zelensky, placing him in the middle of events that led to the impeachment inquiry.

Pence has said that the two did not discuss an investigation into the Bidens, but they did discuss military aid and the issue of corruption in Ukraine. Williams was part of the traveling delegation.

"As President Trump had me make clear, we have great concerns about issues of corruption. And, fortunately, President Zelensky was elected decisively on an anti-corruption message," Pence told reporters a day after the meeting, adding later: "To invest additional taxpayer (money) in Ukraine, the President wants to be assured that those resources are truly making their way to the kind of investments that will contribute to security and stability in Ukraine. And that's an expectation the American people have and the President has expressed very clearly."

He later told reporters at a press availability during an October visit to Iowa that he "never discussed the issue of the Bidens with President Zelensky."

Pence aides maintain that when he went to Warsaw, he was unaware of the Biden allegations being made by the President's lawyer, Rudy Giuliani, despite two of his top aides being present on the July 25 call.

Sondland said he did not remember Pence bringing up aid to Ukraine during that bilateral but he also did not recall whether Pence and Zelensky had a private conversation afterward.

Volker said he "did not get much of a readout at all" on the Warsaw meeting, but the readout he did receive suggested that "it went well" and that "concerning security assistance, the vice president did not have an answer to lifting the hold."

Volker also said that Pence couldn't give Zelensky a date for a White House meeting with Trump.

The top US diplomat in Ukraine Bill Taylor said in his deposition that he received a readout from the September 1 Warsaw meeting from Tim Morrison, a top National Security Council official. Morrison told Taylor that Zelensky "opened the meeting by asking the vice president about security cooperation."

Pence, Taylor said, "did not respond substantively but said he would talk to President Trump that night. The Vice President did say that President Trump wanted the Europeans to do more to support Ukraine and that he wanted Ukrainians to do more to fight corruption."

Taylor told lawmakers he did not know whether Pence spoke with Trump that night.

On September 18, Pence spoke with Zelensky by phone.

"I believe that this phone call was the vice president getting back to President Zelensky to follow up" on topics discussed during their meeting, Volker testified, saying that Taylor briefed him that Pence told Zelensky "security assistance is moving, and we are moving ahead with a White House visit."

Sondland, however, said his team was not "kept in the loop" that the September 18 call between Pence and Zelensky was going to occur and "(was not) asked to listen in."

The transcript

While Pence has attempted to distance himself from the investigation, Trump has only pulled him into the fold.

"I think you should ask for Vice President Pence's conversation, because he had a couple conversations also," Trump told reporters after the release of the rough White House transcript of the July 25 call.

But a transcript of Pence's call has yet to be released and it's unclear if it ever will be.

Pence said in Iowa that he has "no objection" to releasing transcripts of his own calls with Zelensky. "We're discussing that with White House counsel as we speak," he said on October 9.

Officials debated waiting before releasing them for dramatic effect, but they have since put plans to do so on hold. An administration official said that's because they are still being reviewed by White House lawyers, though it only took a matter of days for the White House to review and release the rough transcript of Trump's call with Zelensky.

There had been questions internally about what the point of releasing the transcripts was, given the reaction to the President's own. Officials have downplayed what's in Pence's transcripts, and Williams, who was on the September 18 call, could provide additional details on the conversation.

The fallout

An additional Pence adviser told CNN, "It's a challenging environment," in response to the controversies and scandals the vice president has had to dodge, most recently Ukraine.

The challenge for Pence, as it's always been, is balancing the need to appear loyal to Trump with staying clear of the President's countless controversies.

The plan to protect him, according to sources close to the vice president, is to get him on the road. Pence has traveled frequently in the weeks since the House launched its investigation, from touting the administration's US-Mexico-Canada trade agreement to meeting Turkish President Recep Tayyip Erdogan in Ankara.

And as recently as Tuesday, Pence was giving Senate Republicans advice on how to handle rising questions surrounding the inquiry: read the transcript.

The vice president urged the lawmakers in the room to point out that the Trump administration has provided Ukraine with lethal aid and he advised them to simply refer back to the transcript when facing questions about the President's conduct.