Felix Sater has longstanding ties to the Trump Organization. In 2010 Sater carried a business card identifying him as a senior advisor to Trump. He's quite a colorful fellow. From today's article in The Daily Beast:
Felix Sater is an immigrant who did prison time for stabbing a man in the face with the broken stem of a margarita glass, and he would surely qualify for the label “bad hombre” were he from Mexico instead of Russia.
It was only by becoming a federal informant that Sater avoided an otherwise 20-year mandatory term for a $40 million fraud in which most of the victims were elderly, a number of them Holocaust survivors.
The Russian-born Sater has popped up in the news again recently when the New York Times reported that Sater was involved in hand-delivering a sealed proposal to Michael T. Flynn (who has not yet resigned as national security adviser) "outlining a way for President Trump to lift sanctions against Russia."
From the Daily Beast:
The plan is said by the Times to involve Russia’s withdrawal from Ukraine and a referendum on the fate of occupied Crimea: namely, whether or not the peninsula, which Russian forces seized almost bloodlessly in 2014, would be “leased to Russia for a term of 50 or 100 years” Artemenko reportedly insists that their peace proposal has met with approval among senior aides to Russian President Vladimir Putin.
Sater did not respond to a request for an interview with The Daily Beast. He was quoted elsewhere denying that he had been engaged in actual diplomacy. He did tell Fox News that the effort is just his latest contribution to his adopted land.
“What could be wrong in helping stop a war and trying to achieve peace?” he said. “I have done so much for my country and thought that promoting peace was a good thing.”
Trump has said in interviews (see above video) that he hardly knows Sater and wouldn't recognize Sater if he were in the same room with him, but Sater and Trump have been photographed together numerous times and Sater has traveled with Trump and his children. From The American Interest:
Sater has also testified under oath that he had escorted Donald Trump, Jr. and Ivanka Trump around Moscow in 2006, had met frequently with Donald over several years, and had once flown with him to Colorado.
As for the man Sater stabbed in the face with the broken margarita glass in 1991? Wikipedia says Sater "stabbed the man's cheek and neck with the stem of a margarita glass, breaking his jaw, lacerating his face, and severing nerves. Sater was convicted of first degree assault in 1993 and served a year in prison."
To quote Trump from his 1990 book, Surviving at the Top, "I’ve always been blessed with a kind of intuition about people that allows me to sense who the sleazy guys are, and I stay far away.”