Harbinger: The main antagonist of Mass Effect 2 and Mass Effect 3 (besides Illusive Man as the secondary antagonist), and leader of the Reaper fleet. It is the largest and oldest Reaper in existence and was the controlling force behind the "Collectors"; who were warped versions of the Protheans.

The Catalyst begins with its own creators, the Leviathans, transforming them into the first Reapers and beginning the war its creators had tried to avoid.

The Reapers have existed for at least a billion years, based upon findings on the Leviathan of Dis, an ancient Reaper corpse. Before the Reapers came to be, the galaxy was under the thrall of a race known as Leviathans. They created an "Intelligence" to solve the problem of organics and synthetics killing each other.