Mass Effect, Mass Effect 2 & Mass Effect 3
When the asari discovered the Citadel, they also discovered the keepers, a docile multi-limbed insect race that seemingly exists only to maintain and repair the great Prothean station.
Early attempts to communicate with or study the keepers were failures, and it is now illegal to interfere with or impede keeper activity. Because they are completely non-threatening, keepers have become virtually invisible to everyone else. Similarly, they seem indifferent to other species, except for their tendency to help new arrivals integrate themselves into the Citadel.
No matter how many keepers die due to old age, violence or accident, they maintain a constant number. No one has discovered the source of new keepers, but some hypothesize they are genetic constructs: biological androids created somewhere deep in the inaccessible core of the Citadel itself.