Who is Stephen Hubbard, the American Putin just freed?

Vladimir Putin has released U.S. citizen Stephen Hubbard, who had been convicted by a Russian court on mercenary charges. Putin announced the move at the start of his meeting with U.S. presidential envoy Steve Witkoff in St. Petersburg, saying Hubbard is already at Pulkovo Airport. He will likely leave Russia with Witkoff, who is expected to fly to Oslo later that evening.