5. GLaDOS As A Human
One of the most entertaining parts of Portal 2 is undoubtedly the voice of 'Cave Johnson' (played by the legendary J.K. Simmons) as he directs you through the ruins of Aperture Science's testing areas. Although you neither travel through space or time there's an adventure through three decades of Aperture history as Johnson's narration gives a lot of the insight into the particular companies history during these periods and pieces together a lot of cryptic answers to questions that allow the gamer to 'break the fourth wall' and better understand why certain things are in certain places.
When you first encounter GLaDOS (short for Genetic Lifeform and Disk Operating System) she isn't happy - probably because your character is responsible for her death in the first Portal game - or probably because you are then responsible for her disconnection and resurrection as a potato-battery. In chapter 7, hidden between the first and second orange gel test spheres, is an office with a portrait hanging on the wall. Its a portrait of Johnson, and we've seen lots of photos of the man but this is the only one in which he's joined by his assistant Caroline. http://www.youtube.com/watch?feature=player_embedded&v=cO-DIf3J1pw Upon encountering this photo GLaDOS exclaims that these people look so familiar and if you've managed to put the pieces together by this point you notice you're looking at something far more significant, GLaDOS as a human, before she was uploaded into a robot. There are a lot of theories pointing to Chell's parents being Cave and Caroline and this portrait is a solid piece of photographic evidence showing a distinctive likeness between the trio.