George Papadopoulos "will make a big difference" in special counsel Robert Mueller's Russia probe, the fiancee of the Trump campaign foreign policy advisor told ABC News.
The special counsel's office announced in October that Papadopoulos had pleaded guilty to lying to the FBI. He has since been cooperating with investigators.
"He has sort of been the first domino in Russia-gate," said Papadopoulos' fiancee Simona Mangiante.
Mangiante, an Italian citizen, met Papadopoulos in September 2016 while he was working on the campaign. She said his decision to cooperate with investigators was "brave" and that it placed him on "the right side of history."
He would play a significant role in "clarifying all that's happened," she said.
She also fired back against a claim from former Trump campaign adviser Marc Caputo that Papadopoulos was a lowly "coffee boy."
"There is consistent evidence that he was not a coffee boy," she said. "He set up meetings with leaders all over the world."
She said Papadopoulos, 30, who was based in London during the campaign, was in constant email communication with top Trump advisors including Steve Bannon, who served as chief strategist in the White House, and Michael Flynn, Trump's former national security advisor.
In a tweet, Trump said few people "knew the young, low level volunteer named George."
Trump tweet: ....came to the campaign. Few people knew the young, low level volunteer named George, who has already proven to be a liar. Check the DEMS!
Flynn pleaded guilty last week to lying to the FBI and has been cooperating with the Russia probe.