Keeper

The keepers are a bio-engineered, insectoid race found only on the Citadel. They appear to exist for no other reason than to maintain the Citadel and its systems. The keepers are believed to have been created by the Protheans to serve as caretakers of the station.

Physiology and characteristics
Physically, the keepers resemble large aphids. Little else is known about them other than what can be observed with the naked eye. Attempts to capture a keeper or take it into custody for study will cause the creature to undergo a sudden "self-destruct," with a form of acid being released internally. The affected keeper literally melts into a puddle of proteins and minerals in less than a minute, preventing any serious research. Although it is virtually impossible to study keepers without interfering with them, some enterprising scientists will eventually no doubt find a way. Although purely speculation, perhaps their insectoid metallo-chitinous exterior exhibits some form of shielding from X-rays (or T-rays) as well as whatever more modern scanning methods that have been tried.

The number of keepers present on the station is remarkably constant, although it is unknown where new keepers are born (or produced) to replace those lost. It has been speculated, however, that they may inhabit the inaccessible core of the Citadel, which has yet to be explored.

The keepers themselves reveal nothing of their nature and often undertake tasks that seem bizarre to other races, like rearranging offices or even changing architecture down on the Wards, with no explanation. In fact, all attempts to communicate with them have been in vain. They go about their work, seemingly oblivious to the hustle and bustle going on around them.

Their inorganic components (specifically, the component on the keeper's back and its attached antenna) are speculated to facilitate the coordination between the keepers and the Citadel, though apparently, there is no way currently to communicate directly with the keepers, and no mention is made of any attempts to examine whatever communication or coordination is occurring. Apparently, keepers communicate between themselves with telepathic images, but this might be scientific speculation.

As David Anderson mused in Mass Effect: Revelation, apart from curious scientists (and the occasional inconvenienced citizen who has had their office rearranged) most people on the Citadel stop noticing the keepers soon after they arrive.

Locations
Presidium:  Inside the Citadel Tower At the top level, in the left (facing the council's balcony) wing Inactive, in the open, near the travel terminal, at the base of the stairs to the top level To the left of the inactive keeper, beyond Admiral Kahoku, to the left of a set of benches On the main level, on the right (facing the top level) wing  Outside Citadel Tower, by the Avina. (Avina will interrupt you if you try to interfere with this one.) In the left back room of the Financial District's Emporium On the outer balcony of the Embassy Lounge, around the corner from the door near the barkeep</li> In first room of the wing of the Embassies that also has the lounge, and Executor Pallin's office</li> In the volus and elcor office at the Embassies</li> Opposite the Krogan Monument, near the stairway away from the lake, between the Consort's Chambers and the Financial District</li> Outside the entrance to the Consort's Chambers</li> Outside the Wards Access elevator's entrance, opposite the travel terminal</li> </ol> Wards:  Alone in the back of the small side-room in the Presidium-Wards Access corridor</li> in the red-lit alleyway between Chora's Den and the Presidium Access elevator</li> in the back corner of the upper level of the Markets</li> inside Flux, at the back corner of the Quasar game room</li> behind the Med Clinic entrance on the Upper Wards</li> at the C-Sec Academy entrance</li> behind a barred-off section of the traffic control room in C-Sec Academy</li> at your left in the requisition office in C-Sec Academy</li> in the Normandy's docking bay</li> </ol>

History
The asari first encountered the keepers when they discovered the Citadel.

When the first asari came aboard, the keepers were already there, doing everything in their power to assist the asari in settling onto the station. Ever since, they have gone about the business of maintaining the Citadel in silence as they have for centuries, apparently ever since the Protheans left.

Due to the fact the keepers persist in destroying themselves when interfered with, they are nearly impossible to scan or study. By Council law, it is illegal to interfere with the keepers on penalty of imprisonment, because the Citadel could not be maintained without them.

The keepers are actually the key to the Reaper genocide, which occurs roughly every 50,000 years.

It is later revealed that the keepers, along with the Citadel, pre-existed the Protheans. According to Vigil, the keepers were likely once a race that was either created, conquered, or 'indoctrinated', by the Reapers, possibly even the first race they enslaved. Whatever their origins, the main purpose of the keepers now is to maintain the Citadel in its attractive state so that sentient organic life will settle there, and support their habitation of the Citadel once they take possession of the station, as they have for countless other races throughout time.

However, once an organic species has settled on the Citadel and reached the required level of technological advancement, the Reaper vanguard sends a signal to the keepers compelling them to activate the inactive Citadel relay to dark space, and begin the process of genocide. The Protheans succeeded in altering this reaction to the signal (at the cost of dooming the Prothean species to extinction) so the keepers only respond if the signal comes from inside the Citadel at an unknown location, possibly the inaccessible core. The keepers have changed and evolved so they only respond to the Citadel itself; they are now no longer under Reaper control and pose no threat to anyone.