Boris Epshteyn, who is scheduled to be arraigned Tuesday on election interference charges in Arizona, has played many roles for former President Donald J. Trump.

A college friend of Mr. Trump’s son Eric at Georgetown University, he would become a swaggering TV surrogate for the 2016 Trump campaign before eventually serving as Mr. Trump’s unofficial chief fixer and legal strategist. When Mr. Trump was convicted in New York last month on 34 felony counts, Mr. Epshteyn (pronounced EP-stine) was at his side, huddling with the former president and other aides after the verdict.

He routinely surfaces as a lesser character in Trump-related indictments, court records show. Election cases in Georgia and Wisconsin identify him as “Individual 3” and “Individual A.” In one federal case pending against Mr. Trump, he appears as “Person 5.” In another, his email traffic matches that of “Co-conspirator 6.”

But in Arizona, he is getting a featured role. His indictment there stems from work he did behind the scenes to try to keep Mr. Trump in power after his 2020 election loss. Shepherding a small group of advisers, he helped oversee a plan to deploy fake electors in seven battleground states lost by Mr. Trump, documents show.

“Boris does two things,” said Timothy Parlatore, a lawyer who once represented Mr. Trump but departed largely because of clashes with Mr. Epshteyn. “He coordinates the legal teams and he acts as an emotional support animal to the president. He’s Mr. Good News. He loves telling the president what he wants to hear. And he does that in a way so that he can maintain control over the legal teams — to the president’s detriment, in my opinion.”

Mr. Epshteyn, who plans to plead not guilty, is among 18 people charged in Arizona, including Rudolph W. Giuliani, Mr. Trump’s former personal lawyer, and Mark Meadows, the former White House chief of staff. The defendants all face nine felony counts, related to fraud, forgery and conspiracy.