Dereth was a Vidiian harvester who stole Neelix's lungs in Phage and later helped the Doctor give Neelix a transplant from Kes.
Dereth was one of two Vidiians who ran a hidden organ-harvesting facility inside a planetoid where Voyager's away team was searching for dilithium. Like all Vidiians, he lived with the Phage, a disease that forced his people to take healthy organs and tissue from other species simply to survive.
In Phage, Dereth and his companion Motura were tracked down and captured after Neelix's lungs were removed. When Janeway chose to let them go rather than take the lungs back at the cost of Motura's life, Dereth repaid her by applying Vidiian surgical expertise to transplant one of Kes's lungs into Neelix.