Mortimer Harren was a Starfleet crewman on the Federation starship USS Voyager. Harren was raised on the planet Vico V, a world known for its very celestially-active sky, which may have inspired his intense interest in the field of cosmology (although he denied it). He earned five advanced university degrees in the theoretical study of the subject.
He had an introverted personality, preferring solitude and the academic pursuit of his cosmology theories to socializing. In 2371, he wanted to apply to the Institute of Cosmology on Orion I. However, he needed one year of practical experience in cosmological study in order to be admitted. Since such experience could easily be attained as part of a starship's crew, he signed onto the USS Voyager.
Then, on her shakedown mission, Voyager became stranded in the Delta Quadrant, many tens of thousands of light years from Federation space, by an alien device known as the Caretaker's array. (VOY: "Caretaker") Unable thus to attend the Orion Institute of Cosmology as he had planned, Harren became bitter. He treated other members of the crew with thinly-veiled contempt and avoided any away missions.
He worked in Voyager's engineering department under Lieutenant B'Elanna Torres, the chief engineer. However, when she tried to give him responsibility commensurate with his ability, he refused to do the work, to her great annoyance. She therefore assigned him to the plasma relay room on Deck 15, where his only responsibility was to route power according to Torres' instructions. These instructions were usually carried to him, on a PADD by a crewman. This was the only time anyone interacted with him. This pleased him greatly; he thus was able to spend almost all of his time alone, working on theoretical cosmology calculations, including trying to disprove a standing cosmological theory known as Schlezholt's Theory of Multiple Big Bangs.
In 2376, Seven of Nine, the head of Voyager's astrometrics department, conducted an efficiency analysis of the ship's operations. Her results, presented to the senior staff in a briefing, indicated that Harren and two other crewmen were not performing at acceptable levels, and were reducing the efficiency of the ship's operation. Concerned, Captain Janeway decided to take Harren and the other two crewmen, Tal Celes and William Telfer, on an away mission aboard the Delta Flyer, one of Voyager's shuttlecraft, in hopes of improving their performance. Because of the nature of his academic background, Harren was assigned to look for subspace particle decay, from which new information about star formation could possibly be extrapolated.
During the mission, Harren was unfriendly toward his fellow team members, including Captain Janeway, acting toward her in a manner that fell just short of insubordination. She tried to bond with him but he rejected her attempts.
As the mission proceeded, an unknown entity struck the Flyer, disabling her propulsion system and neutralizing 90% of her antimatter. Harren suggested that the entity was a dark matter proto-comet, something he had theorized existed during his university studies. Since, according to his theory, the proto-comet was attracted to antimatter he urged Janeway to eject the rest of their antimatter to avoid further impacts. However, Janeway felt his theory was still an unproven hypothesis and desired more evidence before taking such a step, which would have left them unable to attain warp speed and return to Voyager.
Later on, William Telfer, another of the three crewmen, was abducted by organisms that were thought to be dark matter lifeforms. Telfer was returned to the Flyer with one of the aliens in his body. This frightened Harren greatly. Soon after, the alien emerged from Telfer and began interacting with the Flyer's environmental systems. Panicked, Harren killed it with a phaser, eliciting an angry rebuke from Janeway, who surmised that the alien meant no harm, but was simply trying to communicate.
More of the aliens attacked the Flyer, presumably in revenge. Janeway ordered the three crewmen to leave in escape pods, but only Harren did so; the others insisted on staying and seeing it through with Janeway. However, in a surprising act, Harren selflessly attempted to draw the advancing aliens to the pod to give the Flyer a chance to escape. However, Janeway took the Flyer to transport range of the pod and had Telfer beam it aboard.