Harbinger

"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 is a Reaper, presumably residing in dark space with the rest of the Reaper fleet.

During the events of Mass Effect 2 Harbinger is controlling the Collectors, through the Collector General, having them abduct the entire population of human colonies in order to use their genetic material to create a Human-Reaper. While controlling the Collector General, Harbinger is capable of possessing any individual Collector, often doing so when the Collectors are engaged in combat with Commander Shepard. While controlling a Collector, Harbinger is able to speak through them, usually to command other Collector troops, or threaten its opponents.

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." Either way, the Collector General is destroyed in the blast.

After the destruction of the Human-Reaper, Harbinger, and the rest of the Reaper fleet, begin their journey from dark space into the Milky Way.

Trivia

 * If Garrus is in the squad while Harbinger is controlling a Collector it will state "Turian, you are considered too... primitive."