Keepers

The keepers were a bio-engineered, insectoid race found only aboard the Citadel, where they maintained the station and its systems in complete silence. Wholly docile and harmless, they appeared to exist for no purpose beyond their endless caretaking, and Council law forbade interfering with them. For centuries the galaxy assumed the Protheans had created them, but the truth was older and far more sinister, tied directly to the Reapers and the cyclical harvest of organic life. (Mass Effect)

Biology#

Physically, the keepers resembled large aphids, and little of their biology was understood beyond what could be seen with the naked eye. Any attempt to capture or study one triggered a sudden self-destruct, in which the creature released a form of internal acid and dissolved into a puddle of proteins and minerals in under a minute, frustrating all serious research. They were mute to the perceptions of other races, though some Citadel scientists speculated they communicated among themselves through telepathic images. Inorganic components on a keeper's back and an attached antenna were thought to coordinate the creatures with the Citadel. Outwardly they sometimes emitted insectoid chirping, but nothing approaching translatable speech. No matter how many keepers died of age, accident, or violence, their numbers remained constant, with no discovered source of replacements, leading some to suspect they were biological constructs grown deep within the inaccessible core of the station itself. (Mass Effect)

Discovery and Early Study#

The asari first encountered the keepers when they discovered the Citadel in 580 BCE. The creatures were already aboard, and they quickly did everything in their power to help the asari settle, behaving like servants surprised by a master's sudden return and scrambling to make everything ready. Ever since, the keepers had gone about maintaining the station in silence, apparently as they had since the Protheans departed. Two scientists, Chorban and Jahleed, took an interest in the keepers and built a scanner to analyze them; though their findings were preliminary, they theorized that the keepers were as old as the Citadel itself. If Shepard aided Chorban's data collection, the scientist later concluded the keepers were engineered to react to a signal recurring roughly every fifty thousand years, made, he believed, by the same builders who created the Reaper Sovereign. (Mass Effect; Mass Effect 2)

True Origin#

The keepers and the Citadel both predated the Protheans, and the keepers were in fact the key to the Reaper genocide that struck each civilization once it grew sufficiently advanced. According to the Prothean virtual intelligence Vigil, the keepers had likely once been a race created, conquered, or indoctrinated by the Reapers, possibly the very first species the Reapers enslaved. Their purpose was to keep the Citadel attractive and functional so that organic life would settle there, and to support that life until the Reapers took possession of the station. Once a resident species reached the required level of advancement, a Reaper vanguard left behind to watch would send a signal that compelled the keepers to open the Citadel relay to dark space and begin the harvest. The Protheans managed to alter the Citadel's signal so the keepers ignored it, but too late to save themselves from extinction. The keepers since evolved to respond only to the Citadel itself, no longer under Reaper control and no longer a threat. Seeing them as useless, Sovereign sought to replace them with a more controllable synthetic race such as the geth. (Mass Effect)

The Reaper War#

The keepers continued their normal routines through most of the Reaper invasion of 2186. Tunnels they used for maintenance served as escape routes for Cerberus forces during an abortive assault on the Citadel. Near the end of the war the Reapers captured the Citadel and moved it over Earth, using parts of the station as a dumping ground for human corpses, and the keepers assisted in processing the bodies. After Saren's earlier attack on the station, the keepers had repaired the damage and even touched up the work done by the station's inhabitants, though no one knew where they obtained their materials. (Mass Effect 2; Mass Effect 3)

Culture#

The keepers revealed nothing of their nature and often undertook tasks that seemed bizarre to other races, rearranging offices or even altering the architecture of the Wards without explanation. All attempts to communicate with them failed, and they went about their work seemingly oblivious to the activity around them. After the Collectors were revealed to be transformed Protheans, members of the Normandy crew speculated that the keepers too might once have been something else before the Reapers reshaped them to serve. (Mass Effect; Mass Effect 2)