Caroline

Caroline is an unseen character mentioned repeatedly by GLaDOS. Caroline was the secretary of Aperture Laboratories's CEO Cave Johnson.

Backstory
Caroline was a former model and poster figure of Aperture Science but as time went by, she became more of Johnson's personal assistant. One day, Cave decided to put his mind and personality into a computer that would run after he died. However, he also decreed that should he die before this takes place, the position would go to Caroline. After Cave's death of kidney failure, Caroline's brain was transferred to the network, which would be infamously known as GLaDOS herself. Over the years, Caroline forgot who she really was and took over her GLaDOS persona, eventually snapping over the surge of emotion from the stress of endless testing.

Caroline herself had supposedly died from a 500m fall. (most likely as the test subject before Chell in the Aperture Science Long Fall Boots video)

Portal 2
While she did not physically appear, she can be heard through audio loops in some chambers and is almost always mentioned by GLaDOS, more over as she bluffs coming back to her former self.

Trivia

 * Caroline is stated to be the test subject who died in the Aperture Science Long Fall Boots ad, as Cave Johnson signaled her name right before the subject started running.