GLaDOS

The number of Aperture Science employees starts as a large group of extremely dedicated scientists, but likely diminishes, until there are only a few of them left. The Aperture Science Handheld Portal Device is likely already completed at that time, but it is unknown if it is before or after GLaDOS' activation.
During that period, Portal Storms continue to rage on Earth and spread chaos, eventually leading to the Seven Hour War, resulting in Earth being defeated and subsequently controlled by the Combine. Later, around 20 years after GLaDOS' activation, Gordon Freeman is awakened from his stasis by the G-Man, and arrives in City 17, which greatly disturbs the Combine's grasp on Earth.

GLaDOS's main purpose of creation is for testing. She was made to try and perfect and get the full potential out of the portal device.