Keeper

Keepers are quadruped insectoid creatures that act as the caretakers of the Citadel in the Mass Effect series. They can be found in various parts of the station, working at computer terminals that only they can use or conducting various retrofitting and construction tasks. They do not communicate with any of the Citadel's residents and are not controlled by the Citadel Council; they were present when the Citadel was first discovered by the Asari and are believed to have been present when the advanced cultures of past 'cycles' took up residence on the station. Since the Citadel was actually built by the Reapers, it is possible that the Keepers were constructed by them or one of the earliest harvested/indoctrinated species from millennia past. Whatever the case, the Keepers aren't telling and it is impossible to communicate with them or really interact with them in any way. Keepers have been known to self-destruct if anyone obstructs them or tries to analyze them, their bodies releasing an internal acid that dissolves them completely. However, the number of Keepers on the Citadel remains constant and new ones are produced in an unknown area of the station that no one - not even the Citadel Council - has access to.