Saren Arterius

Saren Arterius is the main villain of Mass Effect alongside his Reaper flagship Sovereign. He is a turian veteran member of the Spectres, the top operatives of the Citadel Council, who has always been known to be very brutal and ruthless, having little regard for life, and hates humanity, thinking that they should be lower in the hierarchy of galactic society.

History
Saren was once a heroic, but ruthless spectre loyal to the Citadel Council, who did everything necessary to keep the galaxy safe from evil such as the batarians, even if it meant killing people.

Years later, Saren met Sovereign, who was a Reaper, a race of gigantic evil synthetic life forms that terrorized the galaxy millennia ago and anihalated organic life forms. Saren started working with him hoping to overthrow the Citadel Council and also save the current life of the galaxy from the Reaper invasion, by aiding in said invasion by finding the Conduit, which would allow the Reapers to enter the galaxy through the Citadel.

However, after enlisting the aid of a race of AI beings called the Geth and an Asari named Matriarch Benezia, Saren began a murderous killing spree across the galaxy, killing many beings he was sworn to protect. Eventually, he was informed that a Prothean Beacon had emerged from the human colony Eden Prime and Saren led the geth to invade Eden Prime and find the beacon to get information on the Conduit and its whereabouts, as the message the beacon contained could only be read by organics. While there, Saren encountered fellow turian Nihlus Krylik, who was sent to investigate the invasion. When Nihlus questioned Saren of his reason on Eden Prime, Saren assured him he had it under control and killed Nihlus.

Unfortunately, Saren's plan to cover his involvement had failed as the Alliance vessel called the Normandy had responded quickly to the invasion and a vetran human soldier named Commander John Shepard led a squad to stop the invasion and secure the beacon and discovered Saren's crimes. Saren quickly left the planet onboard Sovereign, where Benezia informed Saren that Shepard made contact with the beacon, which made Saren decide to eliminate the human commander.

After Shepard and Saren's rival, Captain David Anderson, reported Saren's treachery to the Citadel Council, the Council refused to believe Saren would do such a crime. During a hearing, Saren attended in hologram and denied murdering Nihlus. He then used his past relationship with Anderson to complicate the matters further. The Council found no evidence to convict Saren and sent Shepard to find evidence while Saren was free to escape justice. Despite multiple attempts to silence Shepard, he met a quarian named Tali, who had a file taken from a disabled geth which proved Saren attacked Eden Prime. With the newfound evidence, the Council revoked Saren's spectre status, but this was not enough to slow Saren's progress.

Meanwhile, Saren attempted to exploit the Rachni by indoctrinating them into his army, and retrieving information on the Thorian, a lifeform with brainwashing powers similar to the Reapers. He also created a "cure" to the krogan genophage in order to control the krogan. Saren believed that with the krogan, he would be unstoppable. Saren established his headquarters on Virmire to house his krogan breeding facility and captured salarian soldiers to indoctricate them.

After being pursued across many planets, Saren finally confronted Shepard in person. During a brief battle, Shepard tried to convince him that he was indoctricated, but Saren refused to believe this. He then escaped Virmire for Ilos before Shepard caused a nuclear blast to destroy his base, at the cost of one of Shepard's teammates (Ashley or Kaidan) that stayed behind to protect the nuclear device.

En route to Ilos, Soverign implants Saren with cybernetic upgrades to 'prevent' Saren from being indoctrinated, while in actuality, ensuring that he remained his pawn. On Ilos, Saren enters an ancient Prothean tomb, with Shepard close behind him. He enters the Conduit, a minature mass relay leading to the Citadel. There, he lets Soverign into the Citadel while he delays Shepard. Shepard can either convince Saren that he is indoctrinated, whereupon he shoots himself, or engage Saren in combat, killing him. However, Saren is not finished. Soverign takes control of Saren's corpse, stripping it of flesh, revealing the heavy cybernetic upgrades beneath. Shepard engages him in combat, destroying the husk for good. As it dissolves, Soverign loses control of it's shields, giving the Citadel defense fleet and the Alliance fleet, including the Normandy a chance to destroy Soverign once and for all.