Jarlath was a long-held captive of the Nyrians who showed Voyager's crew how to move between the habitats.
Jarlath was an alien held for years by the Nyrians in one of the many artificial habitats where they kept the peoples they displaced. After nine years he had found a way to move between environments in secret.
In Displaced, Jarlath entered the habitat holding Voyager's crew and offered to trade. In return for new kinds of food, he showed them how he found the hidden portals. His knowledge allowed B'Elanna Torres to modify the Doctor's mobile emitter to reveal the openings, setting the crew on the path to escape.