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 ImagesPresident 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 YovanovitchFormer 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 ImagesAndriy YermakLev Parnas and Igor FrumanThis 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 APParnas 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 HillFiona 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 ImagesKurt VolkerKurt 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 ImagesGordon SondlandAmbassador Gordon Sondland (C) arrives at the US Capitol October 17, 2019, in Washington, DC.Olivier Douliery | AFP | Getty ImagesBill TaylorCharge 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 ImagesTaylor, 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 KentGeorge 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 ImagesMichael McKinleyMichael 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 ImagesMcKinley 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 CooperCooper serves as the deputy assistant secretary of Defense, where she specializes in Russian, Ukrainian and Eurasian policy. Cooper is scheduled to appear Friday for a closed door deposition in front of the House Intelligence, Foreign Affairs and Oversight committees, where she will be the first witness called from the Pentagon. We’ll learn more about her importance in the matter after she testifies.