Chell

Chell is the main character and test subject in Portal. Since Portal starts with no introduction, cutscenes, or explanation, and you wake up in a rehabilition cell with no backstory, you start the game knowing nothing about who you are, where you are, or what you are doing there. This is probably exactly how Chell herself would feel. Little is known about Chell and her name is not even mentioned by GLaDOS in the game. Instead, anytime she needs to address Chell, instead of by name, GLaDOS will use "" as if the greetings or messages were meant to be filled in with the test subject name and never were.