I don't know about unrealistic but I found GLaDOS a bizarrely sympathetic character considering she has no qualms about killing you. And she does offer cake.

Wolpaw further describes the idea of using cake as the reward came about as "at the beginning of the Portal development process, we sat down as a group to decide what philosopher or school of philosophy our game would be based on. That was followed by about 15 minutes of silence and then someone mentioned that a lot of people like cake."

ETA: For the non spoiler-averse the song from the end credits of Portal gives a pretty good insight into GLaDOS' personality.