Stefan Halper is a notorious FBI informant that spied on the Donald Trump presidential campaign and bragged about his connections to Russian intelligence. Halper's job was to investigate Russian interference in the 2016 election. His job included speaking with Trump campaign officials to find out if they knew of any efforts by Russia to interfere with the election. He then reported what he was told to Stephen M. Somma. It eventually became known that the Trump campaign was aware that Russian hackers had obtained some of Hillary Clinton's emails. Those emails were of course released right before the election and led to Trump's victory.
Donald Trump tried to spin Halper's activities against the Democrats. He called the entire thing "Spygate" and said that Halper was really planted by the Obama Administration to help the Clinton campaign. That theory has since been discredited.
At one point he claimed to have run a successful CIA operation that helped Ronald Reagan win the 1980 election. That activity involved giving the Reagan campaign information about the activities of President Carter. Halper was under the supervision of then CIA Directory George H.W. Bush at the time. Bush of course was Reagan's running mate.
Halper is a former college professor and political scholar. In 2016 he was an emeritus professor at the University of Cambridge where he had also received his PhD.
His exact whereabouts today are unknown. He has not been seen publicly since his name was exposed in 2017. His last known U.S. address was in Virginia where he was listed as a property owner until 2019. He later no-showed when subpoenaed by a Senate committee. He is probably in Europe where he has worked and lived for many years. When the FBI released the Mueller Report in 2019, his name was mysteriously redacted.