Human, you've changed nothing. Your species has the attention of those infinitely your greater. That which you know as Reapers are your salvation through destruction.
~ Harbinger to Commander Shepard

Harbinger is the main antagonist of Mass Effect 2 and one of the primary antagonists in Mass Effect 3 (alongside the Illusive Man). It is the very first Reaper, like Sovereign from the first game, and is the controlling force behind the mysterious aliens known as the Collectors (who are actually indoctrinated and highly modified Protheans). It directly controls the Collector General, and during gameplay it can possess the body of any regular Collector, changing it from a normal grunt into a super-powered enemy with a damage absorbing barrier, enhanced durability, and the ability to fire biotic attacks and fireballs. Harbinger also expresses an interest in Commander Shepard, due to their victory over Sovereign, and often orders the Collectors to capture Shepard alive if possible.

Harbinger's goal is to use the Collectors to kidnap humans and use them to create a Human-Reaper. At the end of the game, Shepard and friends destroy this monstrosity and destroy the Collector base (or alternatively, sterilize it and allow Cerberus to salvage its technology). Harbinger is then forced to lead the Reaper invasion manually by slowly travelling across the Galaxy towards civilization.

In Mass Effect 3, Harbinger is still the leader of the Reapers but has a much smaller role. It is leading the Reapers' invasion of Earth. Its only actual appearance is during the final mission, where it appears behind the beam to the Citadel and personally attacks what remains of Hammer strike force, though the Normandy manages to evacuate Shepard's critically-injured squad. Harbinger fires a powerful laser beam at Shepard which severely injures them, but immediately assumes that the Commander is dead and leaves, allowing Shepard to get to the Citadel beam. Afterwards, Harbinger, along with all other Reapers, is either destroyed or helps to rebuild the galaxy depending on Shepard's final choice.

Harbinger is best known by fans for its famous line "ASSUMING DIRECT CONTROL", which he says sometimes when he takes control of a Collector.