Dejaren was an HD25 Isomorphic Projection aboard a Serosian ship. He acted as cleaner and maintenance unit but was treated badly by the crew and spent most of his time confined to a small area of the ship. He developed a talent for holoprogramming, creating a holographic fish, which he named Spectrum, but he had to keep it hidden from the crew.
In early 2374 his isolation led to his program destabilizing. Having become obsessed with cleanliness to the point of psychosis, he killed the crew and sent a distress call. This was answered by the USS Voyager, and both The Doctor and B'Elanna Torres beamed to the ship. Dejaren quickly became fascinated with the respect and freedom which The Doctor enjoyed on Voyager.
They eventually uncovered Dejaren's crimes when they realized that he was lying about the lower decks; he'd claimed that they were flooded with antimatter radiation, but it was actually where he'd hidden the bodies. Outraged at The Doctor's apparent "betrayal", Dejaren attacked them, severely injuring Lt. Torres by reaching into her chest and damaging her heart, and deactivating The Doctor by swinging a piece of debris at The Doctor's mobile emitter and disabling it. Fortunately, Torres managed to destabilize Dejaren's holomatrix by attacking him with an active isomagnetic conduit. (VOY: "Revulsion")