Few people wield more power in New York City real estate than Andrew Farkas. Mr. Farkas, 65, is the founder of Island Capital, a merchant bank, and has also been a powerful political benefactor, especially to former Gov. Andrew M. Cuomo, to whom he has donated millions and whom he employed in a lucrative role when Mr. Cuomo was out of office. He is also friendly with President Trump and has invested in projects of Jared Kushner, the president's son-in-law. But a 127-slip marina he owned in the U.S. Virgin Islands has put the billionaire real estate investor in a new and troubling spotlight. His co-owner was Jeffrey Epstein, whose private island served as the grim center of a sex-trafficking operation that was just a few miles south of the marina. Mr. Farkas's name appears in Mr. Epstein's personal emails. Mr. Epstein and Mr. Farkas had regular phone calls. Sometimes they met in person for breakfast in New York. And on one occasion, Mr. Epstein even offered the use of Mr. Farkas's plane to visitors in need. Mr. Epstein's stake in the marina was essentially unknown until 2018, when Jennifer Doelling, the chief financial officer of Island Global Yachting, revealed it during a deposition for a tax audit lawsuit. Mr. Epstein was first charged, on a single count of prostitution, in 2006, one year before the marina deal became final, although allegations had been swirling for years. The victim in the charge had been as young as 14 years old.