Michael Piller was a Starfleet officer in the 24th century.
In 2371, he was a crewmember aboard the USS Voyager. He was among the crew stranded in the Delta Quadrant, when Voyager was forcefully brought there by the Caretaker.
In 2374, as a lieutenant commander, he was listed on the crew manifest in 2374. (VOY: "The Killing Game" crew manifest)