In 2376, Harren was one of several officers that Seven of Nine had concluded was not performing at acceptable efficiency levels. In an attempt to resolve the problem, Captain Kathryn Janeway assigned Harren, William Telfer and Tal Celes to accompany her on a scientific mission aboard the Delta Flyer. During the mission, the Delta Flyer was struck by an unknown entity which disabled the shuttlecraft's propulsion systems and neutralized 90% of her antimatter. The ship was then boarded by dark matter lifeforms which scared Harren to such a degree that he shot one of the lifeforms with his phaser which earned him a stern rebuke from Janeway.
When more of the lifeforms boarded the Delta Flyer, Janeway ordered the junior officers into the escape pod, but only Harren obeyed the order. Surprisingly, Harren then used the pod in an attempt to draw the dark matter lifeforms away and sacrifice himself, however, Janeway was able to beam Harren back aboard the Delta Flyer. (VOY episode: "Good Shepherd")
In an alternate timeline where the Voyager crew remained in the Delta Quadrant and formed a new coalition of worlds against the Borg and Species 8472, Harren was killed in early 2374 when Voyager took significant damage following an attack by a Species 8472 battleship. (VOY - Myriad Universes novel: Places of Exile)