For much of Mass Effect 2, the alien race known as The Collectors are assumed to be an autonomous alien species operating on the fringe of the Milky Way, led in total by a Collector General who seems to orchestrate the species' actions from general activity to specific battle schemes. When the Collectors are revealed to be tools of the Reapers, it is also made known that the Collector General's consciousness is totally dominated by a Reaper known only as Harbinger. The General's awareness of this possession is not clear, but the General is completely incapable of autonomous action underneath Harbinger's imposition.
Through the Collector General, Harbinger can possess the consciousness of any single Collector Drone, Assassin, or Guardian at any time, giving the Reaper direct control of the battlefield when necessary. A possessed Collector exudes a fiery electricity, shows increased aggression, utilizes powerful Reaper-infused biotic abilities (including a Dark Energy blast that can jar Shepard from cover), and is imbued with a resilient biotic barrier, making any Harbinger-controlled soldier an exponentially more dangerous presence than most other beings. Moreover, when a Harbinger-possessed Collector is destroyed, Harbinger himself can quickly inhabit the consciousness of any other Collector nearby, and to further support its offensive assault, the Harbinger-Collector is often reinforced by Scions and Praetorians. Harbinger, in battle, often shows an acute interest for Shepard, commanding the Collector troops to preserve Shepard's body, though Harbinger's intentions for such a command remain a mystery.
Mass Effect 2
At the conclusion of Mass Effect 2, when Shepard chooses to either destroy the Collector Base or activate a radiation bomb, Harbinger claims the Reapers will find another means through which to destroy the galaxy's sentient population, and departs from the consciousness of the Collector General. Harbinger itself is shown to be a Reaper very similar in construction to Sovereign. And in the closing image, Harbinger is shown at the apex of the massive Reaper Fleet as the fleet itself begins its march from dark space toward the Milky Way.
Mass Effect 2: Arrival
Harbinger makes a brief appearance at the end of the final Mass Effect 2 DLC, taunting Shepard aboard the Project station, and marking the first time Shepard and Harbinger have conversed with each other directly. The Reaper mocks the tragedy of Shepard's actions--having sacrificed a Batarian solar system to temporarily stymie the Reaper invasion--then reaffirms that no force can resist the Reapers' power.
It is implied throughout the entirety of the DLC that Harbinger is the impetus behind Kenson's betrayal. Via the oppressive Object Rho, Harbinger seemingly possessed Kenson and the science crew behind the Project soon after the Reaper artifact was brought aboard the asteroid, and used an indirect form of Indoctrination to coerce the science team into abandoning the project with hopes of "seeing the Reapers' blessings."
Mass Effect 3
During the Reaper War, Harbinger is one of the Reapers who leads the attack on Earth. Alliance intelligence suggests that Harbinger is the oldest and largest Reaper of the entire fleet.
Harbinger appears during the final battle in London and launches a devastating attack on allied forces attempting to reach the Citadel through the Conduit, using weapons that are considerably faster than other Reapers'. It leaves when it appears that Shepard and the rest of the counterattack have been wiped out. Despite its efforts, Shepard and Anderson make it through, though they are both heavily wounded.
In the end, Harbinger is either destroyed or leaves the galaxy along with the rest of the Reapers.
Mass Effect 3: Leviathan
Since Commander Shepard met Leviathan, it was confirmed that Harbinger was the oldest Reaper.