Then-President-elect Donald Trump meets with former New York City Mayor Rudy Giuliani at the clubhouse of Trump National Golf Club November 20, 2016 in Bedminster, New Jersey. Don Emmert | AFP | Getty Images

President Donald Trump, his lawyer Rudy Giuliani, and Joe and Hunter Biden may be the biggest names in the expanding Ukraine scandal and impeachment inquiry, but plenty of lesser-known players inside and outside government are involved. Here are some of those key players — and why they matter:

Marie Yovanovitch

Former U.S. Ambassador to Ukraine Marie Yovanovitch (C) is surrounded by lawyers, aides and journalists as she arrives at the U.S. Capitol October 11, 2019 in Washington, DC. Chip Somodevilla | Getty Images News | Getty Images

Yovanovitch served as U.S. ambassador to Ukraine from 2016 to 2019. Her tenure was cut short in May, when she was recalled from her post seemingly without cause. Yovanovitch told Congress on Friday she was the victim of a disinformation campaign led by Ukrainians opposed to her efforts to tackle corruption in the country, as well as by Trump and Giuliani, who viewed her as an opponent of their Ukraine agenda.

Andriy Yermak

Yermak, a personal aide to Ukraine President Vlodomyr Zelensky, was the point of contact between the Ukrainian government and Giuliani. Yermak lobbied the president's lawyer to arrange a meeting between Trump and Zelensky in Washington, which Ukrainians hoped would send a clear message to Russia of American support. Giuliani has stated that Yermak expressed an interest in probing the Bidens' dealings in the country.

Lev Parnas and Igor Fruman

This combination of Wednesday, Oct. 9, 2019, photos provided by the Alexandria Sheriff's Office shows booking photos of Lev Parnas, left, and Igor Fruman. Alexandria Sheriff's Office via AP

Parnas and Fruman are Soviet-born, Florida-based businessmen associated with Giuliani. Parnas and Fruman were arrested this month on charges of campaign finance law violations. Both men allegedly funneled foreign money through shell companies to donate to political committees. The businessmen, who were carrying one-way plane tickets out of the country when they were arrested at a D.C.-area airport, were also involved in the effort in Ukraine to spur an investigation into the Bidens, according to Giuliani.

Fiona Hill

Fiona Hill, former Special Assistant to U.S. President Donald Trump and Senior Director for European and Russian Affairs on the United States National Security Council, arrives on Capitol Hill on October 14, 2019 in Washington, DC. Alex Wroblewski | Getty Images

Hill served on the National Security Council as a top advisor on Russian and Ukrainian relations under John Bolton. She testified to Congress behind closed doors on Monday that Giuliani and EU Ambassador Gordon Sondland circumvented the NSC and White House and pursued a shadow policy in Ukraine on behalf of Trump. Hill told lawmakers Bolton sounded alarms about Giuliani's involvement in Ukrainian relations, calling him a "hand grenade."

Kurt Volker

Kurt Volker, the former special envoy to Ukraine, arrives to the deposition related to the House's impeachment inquiry in the Capitol Visitor Center on Wednesday, Oct. 16, 2019. Tom Williams | CQ-Roll Call, Inc. | Getty Images

Volker was the special envoy to Ukraine from 2017 until his resignation in 2019, after he was named in the whistleblower complaint that sparked the House's impeachment inquiry. Volker connected Giuliani with Yermak, and in text messages released by House Democrats, was shown to have worked with Sondland to try to get Ukraine to publicly commit to investigating Hunter Biden.

Gordon Sondland

Ambassador Gordon Sondland (C) arrives at the US Capitol October 17, 2019, in Washington, DC. Olivier Douliery | AFP | Getty Images

A Trump donor who became U.S. ambassador to the European Union, Sondland went under scrutiny after the text messages between him, Volker, Giuliani and acting Ukraine Ambassador Bill Taylor revealed his role in coordinating efforts to pressure Ukrainian officials to investigate Hunter Biden in exchange for military aid. Sondland is set to testify behind closed doors Thursday on Capitol Hill about his role. In a prepared opening statement, he said Trump directed Giuliani to pressure Ukraine to investigate the 2016 election and a natural gas firm tied to Hunter Biden.

Bill Taylor

Charge d'affaires at the U.S. Embassy in Ukraine William Taylor speaks during a press-conference about US-Ukrainian relations, in Kiev, Ukraine, on 27 July 2019. STR | NurPhoto | Getty Images

Taylor, who has served as the acting ambassador to Ukraine following the ouster of Yovanovitch, previously was ambassador to Ukraine from 2006 to 2009. Taylor expressed concern to Sondland that Trump was engaging in a "quid pro quo" with Ukraine for personal political gain. Sondland explicitly denied any appearance of a "quid pro quo." Taylor has left Ukraine and is expected to testify to Congress on Tuesday.

George Kent

George Kent, deputy assistant U.S. secretary of state, arrives for closed-door testimony before House committees on Capitol Hill in Washington, D.C., U.S., on Tuesday, Oct. 15, 2019. Alex Edelman | Bloomberg | Getty Images

Kent, deputy assistant secretary of State for European and Eurasian Affairs, testified on Tuesday to Congress, despite opposition from the White House and State Department. Kent testified that he warned colleagues that Giuliani and his associates were propagating a "fake news driven smear" against Biden and Yovanovitch over Ukraine. Kent also voiced his concerns that he was being pushed out of Ukraine policy in favor of a shadow policy team led by Giuliani.

Michael McKinley

Michael McKinley, a former State Department adviser, arrives to the deposition related to the House's impeachment inquiry in the Capitol Visitor Center on Wednesday, Oct. 16, 2019. Tom Williams | CQ-Roll Call, Inc. | Getty Images

McKinley was a career official within the State Department who resigned on Oct. 10 from his position as senior advisor to Secretary of State Mike Pompeo. McKinley was brought on by Pompeo to help restore morale following Rex Tillerson's ouster as secretary of State, who was widely unpopular within the department. However, McKinley told members of Congress on Wednesday that he resigned due to the State Department's reluctance to defend career officials from political attacks.

Laura Cooper