Physically, they were humanoids who released a unique pheromone. This affected other species, preventing them from remembering encounters with the Ramurans for more than a few hours. Due to technological enhancements, they were also immune to most forms of scanning technology (including tricorders).
They supplemented their naturally understated nature with technology, rendering them practically unknown to the surrounding civilizations. Their ships and personnel used cloaking devices and they used computer viruses to wipe any records of their existence from alien computers.
As a consequence of their obsession with maintaining a covert society, it was illegal for a Ramuran to try to leave. Any who do were hunted by the tracers. This force often acted in secret but worked with others if forced, relying on their technology and natural ability to fade from memory. Once a fugitive was captured, they used a neurolytic emitter to remove the subject's memories of the outside world.
They were first encountered by the USS Voyager in 2374. A tracer named Kellin crept on board the ship while hunting a fugitive but was discovered by Commander Chakotay. Having explained her situation, the crew aided in the capture of the escaped Ramuran. During this time Kellin and Chakotay developed a relationship but when her mission was complete, Kellin left the ship and faded from memory.
Unable to stop thinking of Chakotay, she returned a month later and requested political asylum on the ship. This was duly granted and Chakotay began to love her again, despite not remembering their previous encounter. Sadly she was found by another tracer named Curneth who wiped her memory. Unable to understand why she wished to leave Ramura, she left for good.