Physically, the race are humanoid, although they have a pale, almost metallic cast to their skin. They use holographic technology and create sentient holograms to act as cleaners and maintenance personnel aboard their ships. Unfortunately, while these programs, referred to as isomorphs, are given individuality to the point where each is given their own name, they are not well-treated by their creators, who view them as little more than tools.
In 2374 the USS Voyager received a distress call from a Serosian hologram called Dejaren. Although he claimed the crew of his ship had been killed by a virus, it later became apparent that his program had destabilized and he had become obsessed with cleanliness to the point of psychosis. As a result he had murdered the crew, disgusted at their organic nature. (VOY: "Revulsion")
No Serosians were ever named on-screen, apart from the holographic Dejaren.