Saren Arterius
Saren is the bad guy. He has a deep resentment towards humans because he lost his brother in the First Contact War against them, and he does everything he can to keep them out of the Council. He was a brutal Spectre, torturing and killing any who got in between him and his objective. His reputation for being ruthless preceeded him but he always got the job done and was favored by the Council for his deeds.
However, Saren stumbled upon a little secret about those that came before. He found out that the Protheans where wiped out by an ancient race of machines known as the Reapers. Saren became obsessed and tried to find a way to delay the Reapers return, but his ship, a Reaper, eventually managed manipulate Saren using subliminal messages and other brainwashing techniques to actually work for the return of the Reapers. Saren became a slave, and a dangerous one at that.
Because of Saren's relations with the Reapers, whom the Geth venerate, he managed to control them and became accepted as their leader. He used them to secure certain points of interest to help him locate a lost Prothean version of a Mass Relay named the Conduit. With it he intended to jump himself onto the Citadel and activate it to allow the rest of the Reapers through from Dark Space.
Saren's plan went well, as he did manage to activate the Citadel, but Commander Shepard managed to convince Saren that he was in fact being manipulated by the Reapers. Due to this realization, Saren, resigned to his fate, shot himself dead seeing it as the only way to redeem himself for his actions.
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