Saren Arterius is a former turian Spectre and the main antagonist of the first game. He is known for 'getting results' by any means necessary. More often than not, this meant he killed everyone and everything in range—the target, any witnesses, and innocent bystanders as well. In the novel Mass Effect: Revelation, he tortures and kills his victims mercilessly, rarely having any regards for sentient life or the lives of anyone in his way from getting the job done, even deliberately killing innocent bystanders and witnesses (after he had extracted key information from them). Saren himself does not consider his methods harsh; for example, when he brutally tortures a batarian in the novel until he has been exhausted of information, Saren snaps the batarian's neck while he is unconscious and claims that it was an act of mercy and that he isn't "a monster." His seething hatred for humans is speculated by other characters in the novel to stem from the loss of his brother in the First Contact War.

After the events described in the novel, and before the start of the first Mass Effect video game, Saren takes command of an army of geth through the use of a mysterious artifact known as Sovereign, an ancient ship that supposedly predates the Protheans. With a machine army at his command, he uses them to both do his bidding and oppose the efforts and actions of the player across the story of the Mass Effect video game. One of his main goals at first appears to be to unleash the genocidal Reapers upon the galaxy once again to take revenge on humanity. In reality, he is merely assisting Sovereign in bringing the rest of the Reapers into the galaxy to prove that organics are worth enslaving, rather than exterminating.

In the game it is revealed that Saren had known about the Reaper threat and had independently tried to discover a means to stop it. It is unknown how Saren found Sovereign, however, Sovereign slowly began to manipulate Saren through subtle subliminal messages into helping him into bringing back the Reaper threat. During the Geth invasion of the Citadel, Saren uses the Prothean Conduit to gain access to the Citadel to allow Sovereign to activate the Citadel Mass Relay. The player can then either attempt to kill Saren before he can activate the relay or appeal to Saren to fight his indoctrination, prompting him to commit suicide. Either way, Shepard later has one of his/her crew members shoot Saren a final time in the head to make sure he's dead, but he is still revived and controlled by Sovereign through his grafted cybernetics, transforming him into a powerful enemy who engages Shepard and his crew in one final battle. Upon defeating the Sovereign-manipulated Saren, Sovereign itself loses its defenses, allowing the Citadel Fleet and the Alliance Navy to destroy it. Saren is voiced by Fred Tatasciore.