Keepers are a bio-engineered race found only on the citadel and are completely harmless. It seems as if there only purpose is to maintain the Citadel and its systems. They are belived to have been created by the Protheans to take care and operate the citadel. The keepers were said to be indoctrinated or enslaved by the repearpers according to Vigil. The keepers aid the reapers in their genocides by opening the relay to dar space when given a signal from the reapears. This allows the reapers to enter the galaxy and wiped out and enslave all life. They only are given the signal when a civilization reaches its peak power level. In mass effect when commander shepard scans the keepers he learns that the reapers were engineered millions of years ago by the same people that made soverign and probably all the reapers. When the reapers were destroyed or controlled any surviving keepers ( if any ) would only stay in existence to operate the citadel controls.