Saren was the villain and antagonist in Mass Effect 1. 


Saren was a specture that developed a hatred for humans after his brother was killed by humans in the First Contact War. He decided to serve the Reapers because he felt that no one was able to destroy them and many lives would be spared if the surrendered. For this purpose he served the reapers. saren is eventually kileld by Commader Shepard.