Saren Arterius was a Turian and a legendary member of the Spectres, who are The Citadel's Elite Special operations forces. Due to the fact the Spectre's are a classified unit, his records are sealed. However a few facts are known about his past, It is known that he followed the Turian tradition of enlisting in the military and entered military service at the age of 15. After just one year of training, he was attached to an active combat unit; it is unknown if he or his unit took part in the first contact war with humans but his impeccable service record garnered him a spot in the Spectre's at age of 20; something that was virtually unheard of. Nevertheless, The Council's faith in him was well placed, as he was ruthless and accomplished every single mission that the council had assigned to him and to his credit he quickly developed a reputation for ruthless efficiency. There were a number of disturbing rumors about the brutality of his methods, there was no denying the results he produced and thus he became one of the council's most cherished and trusted instruments. In the first contact war, he suffered a personal tragedy, the loss of his brother. Since this event, Saren has held a grudge against humanity and the expansion of their kind. He pointedly refuses to acknowledge any gains that humanity has made and views them as an affront. He seeks to put humanity in its place so to speak. During the events of Mass Effect: Revelation, he undermines Captain Anderson's quest to join the spectres, by sabotaging the mission and killing hundreds using explosives.
He blames the entire incident on Anderson and he uses it as an excuse to point out that Humanity is not ready and would not be for some more time to come. In the book, his "interrogation" methods are explained in quite a graphic detail as he patiently tortures a Batarian by slowly breaking his fingers and by poking out his eyes. In that sense, it is shown that Saren is a ruthless person who would not stop until his mission is completed. At the end of the book, he learns about Sovereign from a doctor before killing him. His intelligence is shown here as he correctly assumes that the doctor has been slowly falling "under the control" of sovereign and realizes that he needs to take safeguards to protect himself from it. However, he thinks that Sovereign is a weapon that can be used to control the Geth and put humanity in its place and elevates the Turian empire.
It is unknown what Saren was doing during the time between the end of the book to the events of the first Mass Effect game. However, it is assumed that he remained loyal to The Council as they still hold him in high esteem. He is in his early 40's now and he shows no signs of slowing down, this is evident from the fact that he seems to be actively involved in the Spectre program and he even befriended a fellow Turian name Nihlus. Nihlus is considered his protege and Saren puts in a recommendation with the Council for his advancement. Saren appears in the first level of the game and it is evident that he has changed. Without remorse, he shoots Nihlus in the back of the head when the latter is talking to him and calmly proceeds to order the Geth to "set the charges and destroy the entire colony". Worthy of note is the fact that he has multiple cybernetic implants running throughout his body. In a sense, he seems less organic than the other Turians, his armor is bulky and is also interlaced with wires and mechanism. After Commander Shepard proves Saren's involvement in the assault on Eden Prime the council revokes his status as a Spectre, however due to his decades as a Spectre, he has a lot of useful contacts both within The Citadel and outside it and as such he is able to keep tabs on everything that is going on in citadel space. He also gathers around him powerful allies like Matriarch Benezia and with Sovereign 's indoctrination is able to turn them all into his puppets that without question do his deed. The player's first real encounter with Saren happens on Virmire after Shepard had managed to outwit the Geth and shut down the facility.
Saren is all but unstoppable, as his armor seems decades ahead of the current crop. However, Shepard is able to overpower Saren, and with Sovereign's imminent approach both he and Shepard leave. After learning about the Conduit, Shepard follows Saren to Ilos and then finally to The Citadel. Here, Saren is trying to open the Citadel Gates so that Sovereign can open the Mass Effect Relays and unleash the Reaper fleet on an unsuspecting Galaxy. Saren admits that he has let Sovereign implant him in order for him to better challenge Shepard. He claims that it is too late for him to stop what is happening and that Sovereign is too powerful for him to fight. Shepard can convince Saren to see the error of his ways and if he is successful, Saren will shoot himself in the head thereby freeing him of the Prison; the Prison that his body had become. His final words are "Thank you Shepard" This shows his remarkable will, as he is one of the few people who after being in Sovereign's service for so long can successfully fight off the effects of indoctrination. Saren's body is reanimated as a hybrid Geth hopper and once the player defeats it, the remains incinerate.