Keeper

The Keepers are a bio-engineered race native to the Citadel Station. They are believed to have been created by the Protheans themselves, and they serve as maintainers and caretakers of the station.

History
Little is known about the Keepers up until the Asari encountered them when they discovered the Citadel. When the first Asari came aboard, the Keepers were there, doing everything in their power to assist the Asari in learning about the station and it's operations. Ever since, they have gone about the business of maintaining the station in silence.

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, as any attempt to capture or study a Keeper causes it to release a potent acid, completely destroying the Keeper, and preventing any serious research.

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 core of Citadel Station, which has yet to be explored.

The Keepers themselves reveal nothing of their nature. 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.