Ramurans produced a sort of pheromone which blocked the long term memory engrams of others. As a result, memories of Ramurans started to fade within hours and would be completely forgotten by the next day. Other biological characteristics included the presence of a tibia, and a need for sleep.
They had also developed technology to enhance their covert existence. They were impervious to technology like tricorders and transporter locks, and might implant computer viruses to wipe any records of their existence from alien computers. Furthermore, their ships and personnel used cloaking devices based on a sophisticated polarization technique. They did not answer hails.
Ramura was a closed society, and they only infrequently encountered other species. One of the most important edicts in Ramuran law was that no one was allowed to leave or reveal anything about them to the outside world. One Ramuran described his society as strong and cohesive. Their population was in the millions.
Those who did attempt to leave were hunted by the tracers, a kind of bounty hunter. At least some of them held high security clearances. Once a fugitive was captured, they used a neurolytic emitter to remove the subject's memories of the outside world. To the knowledge of one tracer, no one had ever successfully gotten away.
They were 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, which lasted several weeks, 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 about 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 Voyager for good.