Mass Effect: Blackwell

It is unsure what the keepers really are. Some theories insist that the keepers were created by the Protheans. But the Prothean VI, Vigil, said that this was untrue, and that the keepers were most likely the first race to inhabit the citadel before the Reapers came through and turned them into the mindless biological androids that they are. Continuing to lurk around the citadel, and work on the citadel. They are said to activate the citadel when the Reapers send them the signal, allowing Reaper ships to come through and take over the Galaxy.

The Keepers are kept in the Citadel for maintenence reasons. As the Citadel is not entirely understood, the Keepers are needed to make basic repairs. This fits in with the Reapers’ plans of preventing complete understanding of the Mass Effect technology.

It is currently impossible to communicate with the keepers. Whenever a keeper is taken into custody for study, the keeper “self-destructs”. An acid is released inside the body, turning it into a puddle of proteins and minerals in less than a minute.