Dejaren was an HD25 Isomorphic Projection with extreme hazard clearance, serving aboard a Serosian vessel in 2374. His responsibilities there included cleaning the reactor core and ejecting antimatter waste. However, he claimed to have limited knowledge of the ship's controls, and to not be designed for medical functions.
He 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. Eventually, he started to see himself as being treated badly by the crew, to the point of developing hostility and disgust towards organics, and seeing himself as a slave. He at one point expressed a desire to explore the galaxy.
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 the Voyager.
They eventually uncovered Dejaren's crimes when they realized that he was lying about the lower deck aboard the Serosian ship; 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 Torres by reaching into her chest and perforating her heart ventricle, and deactivating The Doctor by swinging a piece of debris at The Doctor's mobile emitter and disabling it.