Mass Effect 2's Collectors are the main antagonists of the series installment—or are they?
The Collectors are a strange, rarely seen race from beyond the edges of Terminus Systems. They almost never enter Citadel Space—making them nothing more than a scary story to most citizens. However, the Collectors are quite real, and tend to appear in and around Omega via the Omega 4 Relay.
This species gets their name from their strange "trade" requests. The Collectors will occasionally put out requests for particularly specific individuals—sometimes in sets of a dozen or more. What they do with these collections is unknown.
There are spoilers for the ending of Mass Effect 2 beyond this point. Players who wish to remain unspoiled should click away now.
Mass Effect 2: Collectors Explained
Commander Shepard first encounters the Collectors in an ambush. A Collector ship rips the Alliance-built Normandy apart, shooting Shepard into space and killing them. During Shepard's two-year absence and revival thanks to Cerberus and the Lazarus Project, the Collectors begin to target and "collect" human colonists.
It's later revealed that the Collectors attempted to purchase Shepard's body—an endeavor stopped by Dr. Liara T'Soni and a drell known as Feron.
The Collectors operate like a collection of unthinking, unfeeling drones piloted by a single central hub of consciousness. In Mass Effect 2, Shepard and the Normandy crew discover exactly why this is.
The Illusive Man will eventually contact Shepard a few main missions into the game and send them to a "derelict" Collector ship. While exploring the ship, the Normandy's new AI system, EDI, will begin analyzing the interior and exterior of the environment. The team quickly learns that they Collectors share a similar "quad-strand genetic structure" previously exhibited by only one other species to inhabit Citadel Space: the Protheans.
Shepard connects the dots and comes to the conclusion that the Collectors are what was left of the Protheans after the Reapers swept through and eradicated them. According to EDI, the DNA shows an "extensive genetic rewrite," explaining how the Collectors gained their distinct appearance as a sort of artificial "evolution" of the Protheans.
Later, in a conversation with salarian scientist Mordin Solus onboard the Normandy, Shepard learns exactly what happened to the species. The Reapers collected and indoctrinated them which, as noted in Mass Effect 1, led to a decline in their cognitive abilities. The Reapers then began to replace certain parts of their bodies with technology to compensate for their deterioration.
According to Mordin, the Collectors are "closer to husks than slaves" with most of the major organs and processes replaced with cybernetics. He points out that they barely have any concept of consciousness, culture, or even things as inherent to an individual as identity and gender. Everything that makes a being organic has been "replaced by tech." The Collectors are unaware tools for the Reapers to use to further their own ends and nothing more—completely beyond saving.
Shepard goes on to assault the Collector Base where they discover the Collectors are attempting to use the human genome to construct a human-reaper hybrid. The commander manages to destroy this hybrid and blow up the base in one operation. However, this doesn't wipe out the species completely.
Mass Effect 3
The Collectors make a return on behalf of the Reapers in Mass Effect 3's Reaper Invasion. They bolster Reaper forces, using their Seeker swarms against other races—most notably during the fight for the turian planet Palaven.
Another race, the Leviathans, are able to free some Collectors from Reaper control during the conflict. Most Collectors wither away without Reaper input while a select few are able to "awaken" and go on to fight against the Reapers to avenge the Protheans.