Jarlath was a prisoner held captive aboard the Nyrian biosphere vessel.
His people were being held captive in a desert environment which was similar to the habitat on their homeworld. He was able to get in contact with the crew of USS Voyager through one of the portals between different habitats. He wished to trade with the crew and explained that the previous inhabitants were not much for bartering. He told them that the former inhabitants had died from a plague.
His visit gave the crew an idea of using the portals to escape, although he explained that it had taken him almost nine years to find just one portal. After Kathryn Janeway traded him a couple baskets of fruit, Jarlath helped B'Elanna Torres locate a transmitter that was used to allow The Doctor's optical sensors to detect the microwave signature of the portals. As the crew transversed the portals, Jarlath went with them, although he was captured by the Nyrian guards and sent back to his habitat.
Jarlath and his people were freed after the crew of Voyager took control of the translocator device and threatened the Nyrians with transportation to an icy cold habitat which was detrimental to the Nyrians' physiology. (VOY: "Displaced")