The Mass Effect trilogy has many compelling characters, but few as mysterious as the Illusive Man. He first appears shrouded in darkness, presiding over the organization known as Cerberus and claiming to be the one responsible for bringing Commander Shepard back to life after their violent death at the hands of the Collectors in the prologue of Mass Effect 2. He claims to be working in the best interest of humanity, and revives Shepard so they can confront the real threat to galactic life which is being ignored by the Citadel Council: the Reapers.
The origins of the Illusive Man begin in the time before the first Mass Effect. Previously known by the name Jack Harper, the Illusive Man served in the First War against the Turians, fighting on the planet Shanxi. In the midst of the war, Harper comes into with an unknown artifact with indoctrination properties. He becomes embroiled in a Turian plot to use the artifact which unfolds after the short First War has ended, and is one of the first Humans to set foot on the Turian home world Palaven. Catastrophic events transpire as the result of an attempt to use the artifact, and Harper barely leaves Palaven alive. The events have a profound effect on Harper, and he begins work on a manifesto, claiming Humanity would inevitably be the target of a genocide from one of the recently discovered alien species. The work was dismissed by the media at large as being human-centric, survivalist drivel written by an illusive man.
Taking his place as the leader of Mass Effect's Cerberus organization, the Illusive Man begins working to accomplish what he sees as Humanity's ultimate goal - to establish its rightful dominance over the other interstellar species. The Illusive Man was not featured in the original Mass Effect, but his backstory and exploits during the first game were canonized in a series of novels and comic books. While compiling his vast network of informants and building the paramilitary strength of Cerberus, the Illusive Man comes to recognize Commander Shepard's importance in the future of Humankind.
The Illusive Man is Mass Effect's Greatest Terrorist
While the Illusive Man claims to be working in the best interest of the Human race, his radical exploits become more apparent throughout the events of the Mass Effect trilogy. Seeking to make Humans the dominant race in the Milky Way, the Illusive Man uses any means necessary, including the assassinations of high-profile politicians and religious leaders on Earth. He also orchestrates the destruction of multiple Alliance freighters carrying Element Zero, ensuring large populations are exposed to "eezo" dust, thus facilitating the birth of more Humans with biotic aptitude - a step he considers essential in elevating Humanity.
When the Reapers become an apparent threat, and the sophistication of their technology is recognized, the Illusive Man seeks to harness that power for the benefit of Humanity. His hubris leads to his own indoctrination and ultimate destruction. The man who first appears to Commander Shepard as a sympathetic compatriot against the Reaper threat is quickly revealed to be the xenophobic, manipulative narcissist he is. The Illusive Man's inclusion in the ambitious plot of Mass Effect 2 is a major component in the series' universe achieving its nuance. His glowing eyes and domineering silhouette contribute the atmosphere of Mass Effect, while his character helps bring to light a darker side of Humanity present in the Milky Way.