You are Alfred(Phrédo)Robrtson, an elictrical engineer who helped to construct the Aperture power-grid as well as wire GLaDOS into it. Kept on-site in case of emergency you were forgotten by the now deceased employees as well as GLaDOS*. You are woken up by your containment unit in its final moments of back-up battery life and you alone explore and escape while finding clues piece by piece about the past and present of Aperture Science. You are the Escapist.
Don't judge the skybox, cubemaps, or lack of optimizing. It's still heavily under construction. By secretly making certain surfaces unportalable makes traveling around a lot harder than it looks. Also; don't fall.