Harbinger

Harbinger is a Reaper who resides in dark space with the rest of the Reaper fleet. It is the overseer of the Collectors' operations, frequently possessing individual Collectors to fight battles personally. Harbinger's involvement first began in Mass Effect: Redemption, when it made a deal with the Shadow Broker to gain possession of Commander Shepard's body. This plan ultimately failed, and Shepard was resurrected by the Lazarus Project.

Harbinger is voiced by Keith Szarabajka.

Mass Effect 2
During the events of Mass Effect 2 Harbinger is the mastermind behind the Collectors, permanently possessing the Collector General. It directs the Collectors to abduct the entire population of human colonies in order to process captives and use them to create a Human-Reaper.

While controlling the Collector General, Harbinger is capable of "assuming direct control" of any individual Collector, often doing so when the Collectors are engaged in combat with Shepard. While controlling a Collector, Harbinger is able to speak through them, usually to command other Collector troops, or threaten its opponents. However, the process of possession appears painful or traumatic to the possessed Collector, and it always disintegrates when defeated. This effect is very similar to Sovereign's possession of Saren Arterius's corpse.

Harbinger shows a personal interest in Shepard, particularly a continuing interest in recovering the Commander's body, as it orders the Collectors to recover Shepard's corpse intact if possible. This could be another consequence of Shepard destroying Sovereign, and drawing the Reapers' personal attention as a result, or relate to the Reapers' orders to gather human genetic material.

After Commander Shepard's team assaults the Collector Base and destroys the still incomplete Human-Reaper, Shepard can either destroy the base completely, or purge the base of all life. Both options will result in Harbinger relinquishing control of the Collector General and vows that the Reapers will "find another way". Harbinger is then seen with the rest of the Reapers in dark space, moving toward the Milky Way galaxy.

At the conclusion of Mass Effect 2, Shepard appears to be aware of Harbinger's personal involvement with the Collectors, as the Commander specifically mentions it to the Illusive Man, as well as Liara T'Soni during the conclusion of the Lair of the Shadow Broker (mission).

Arrival
At the end of Arrival, Harbinger appears via a hologram to have a brief conversation with Shepard about the futility of the Commander's fight against the Reapers. However, if the assignment is completed before the Suicide Mission, it will speak through the Collector General instead.

Mass Effect 3
Harbinger appears during the final battle, trying to prevent Shepard from reaching a Conduit beam leading to the Citadel, killing two squadmates. Despite its efforts, Shepard and David Anderson make it through.

It may be notable that Harbinger seems to fire at a much higher rate than other Reapers.

Trivia

 * Keith Szarabajka, who voices Harbinger, Delan, Lilihierax, and Elias Kelham, also provided the voices for Nug Wrangler Boermor and Herbalist Widron in BioWare's Dragon Age: Origins and Bartrand Tethras in Dragon Age II.