Zeus is the main antagonist of the series. Considered to be the best hacker in the world and a closely observed subject by both NSA and CIA, and the archenemy of Marco. The two were apparently rivals in their past, until Zeus broke into the CIA mainframe and then framed Marco for the crime, escaping soon after.
At some point he was approached by Coral Vega and made security chief for Venus Gate's labs, especially Level 4, for which he created the tight security systems. A deeply disturbed person, he saw war, strife and conflict as a boring, unrealistic form of entertainment, and claimed that humanity as a species had lost its will to survive.
Keeping such ideas a secret from his employers, he devised a way to control Medusa carriers at will, applying it to Shizuku when he found she was much more powerful a carrier than Alice herself. His last act as a human being was amusing himself while looking at Shizuku creating deformed aberrations, before turning his own self into data and entwining it with the monster Shizuku would later become.
Zeus was only seen or mentioned in flashbacks during the course of the story until the end, where he presented himself and declared his will to use Shizuku, a never-ending supply of creatures defying the laws of nature, to build up his own army of beastmen warriors and other fantasy-like beings and with them plunder the world into chaos, an endless game of survival for his own amusement. To do so, and knowing that Shizuku would wake up her sister from her induced slumber, he tampered the other survivors' minds, forcing them to unconsciously not run away after waking up, and especially protect Kasumi's life, which was necessary to coerce Shizuku in doing his biddings.
His material body reduced to a rotting corpse, Zeus manifested himself through membranes expelled by Shizuku's tail, and later took direct control of her, declaring the act to be "the birth of a god". This final act proved to be fatal, as he was first weakened when Kasumi and Shizuku encountered each other, and then completely erased when Marco hacked into his data, deleting him.