Mass Effect 2
Immediately following the First Contact War, an anonymous extranet manifesto warned that an alien attempt at human genocide was inevitable. The manifesto called for an army -- a Cerberus to guard against invasion through the Charon relay.
Derided as "survivalist rhetoric written by an illusive man," the manifesto and its anonymous author soon fell off the media radar. But in 2165, terrorists stole antimatter from the SSV Geneva; the sole figure arrested named his sponsor "Cerberus." Throughout the 2160s and 2170s, alleged Cerberus agents assassinated politicians, sabotaged starships bearing eezo, and conducted nightmarish experiments on aliens and humans. Denounced as human-supremacist, Cerberus calls itself human-survivalist.
Counterterror experts speculate Cerberus may have changed leadership with its recent shift to stockpiling ships, agents, and weapons. Whether "he," "she," or "they," the Illusive Man hides his finances behind shell companies. Few doubt he will kill anyone attempting to expose him.
Mass Effect 3
In recent years, the pro-human syndicate known as Cerberus has seen its influence grow galaxy-wide. The largely untraceable organization now includes private intelligence agencies, biotics laboratories, research facilities, and the lucrative corporations that provide a front for it all.
Cerberus's charismatic leader, known only as the Illusive Man, drives the organization's philosophy and interests. The level of secrecy he maintains puts professional intelligence agencies to shame.
As Cerberus grows, so too does public distrust of the organization. Some commentators have remarked that Cerberus is not so much pro-human as it is anti-alien. Others question the blind loyalty of its employees.