A tracer was a highly trained Ramuran agent assigned to track down and retrieve those who attempt to leave their homeworld. They hid themselves and their ships with polarization cloaks, and they used computer viruses to eliminate any records of their presence. They were armed with neurolytic emitters, with which they erased the memories of their targets, making them more cooperative. Tracers were extremely persistent, for the disgrace of returning home empty-handed would be too great to bear.
Kellin was a tracer who tracked the fugitive Reskat to USS Voyager in 2374. After she fulfilled her mission, she found that she had developed feelings for Commander Chakotay and returned to Voyager as a fugitive herself. She was hunted by more tracers, one of whom, Curneth, infiltrated Voyager and erased her memories. As a result, Kellin returned willingly with him to Ramura. (VOY: "Unforgettable")