The Mass Effect series is filled with many strange alien creatures, many of which are sapient and part of a vast and complicated galactic community. There is one alien race, though, that is integral to the Mass Effect story yet does not directly interact with others in any way - the Citadel's Keepers. These four-legged, insect-like creatures go about their business of maintaining the Citadel space station in almost complete mystery.
The current occupant species of the Citadel during the original Mass Effect trilogy believe the Keepers to be a bio-engineered creation of the technologically advanced Protheans, who previously occupied the station. This is merely an assumption, however. The Keepers have proven impossible to study, since capturing one results in the Keeper melting itself from the inside with a corrosive acid. The Keepers appear to do only one thing: tend to the many complex systems of the Citadel.
The population size of the Keepers is unknown, but their work appears to be instrumental in maintaining the space station, which serves as the capital for the Citadel Council. They navigate the massive structure through tunnels and are able to venture into otherwise inaccessible and unmapped locations. Since so much of their important routine remains a mystery, it is against Citadel Security regulations to interfere with a Keeper in any way.
Mass Effect: The True Origins Of The Citadel's Keepers
[Warning: SPOILERS for the Mass Effect trilogy follow.]
Contrary to the theories of Citadel citizens, the Keepers predate even the Protheans. The Prothean virtual intelligence known as Vigil, found by Commander Shepard on Ilos, reveals the Keepers were the caretakers of the Citadel when the Protheans found it as well. The Keepers are actually agents of the Reapers, though their exact origins are unknown. Vigil hypothesizes they may have been created by the Reapers long ago or were simply an early species to be indoctrinated by the genocidal machines.
Although the inception of the Keepers remains a mystery, their role in the Reapers' harvest is explained by Vigil. The Citadel, designed to lure sentient species out by providing a hub for interstellar civilization, is maintained by the Keepers so that its pristine condition is too good to pass up. Once the vanguard Reaper left behind in the Milky Way determines galactic life is ready for harvest, it attempts to signal the Keepers to activate the Mass Relay that lies dormant in the Citadel.
The Protheans succeeded in altering this signal, but not in time to save the Protheans themselves. Without this prerogative, the Keepers were useless to Sovereign in the first Mass Effect and kept about their janitorial duties, though they still assisted the Reapers in the disposal of bodies during the Reaper War in Mass Effect 3. While a mysterious and mostly docile alien race, the Keepers play a complex and important role the overarching story of Mass Effect - a role that is subject to the will of various other species.