Chell is the main character of the Portal series. She is a test subject in Aperture Laboratories. Chell is the only test subject, and the only live human, in the game series. Little is known about her history before being a test subject. She is mute, and never speaks throughout the game. Chell eventually escapes Aperture at the end of Portal 2, experiencing the outdoors for the first time.
GLaDOS is the AI system that runs Aperture Laboratories. GLaDOS is an abrreviation of Genetic Lifeform and Disk Operating System. She has complete control over the facility. She has a particular distaste for Chell, and makes sarcastic comments to make her life unpleasant. She is defeated at the end of Portal 1, but is revived in 2. GLaDOS ends up becoming less hostile towards Chell when Wheatley takes over, as he turned her into a potato battery.
Wheatley is a maintenance robot in Portal 2. He leads Chell through the ruins of Aperture, offering compaionship and assistance. He often acts afraid of his superiors, claiming that, "They told me, if I ever turn this thing on, I would die!" He ends up taking control of Aperture, sending it into a far worse state than before. He turns out to be a robot designed to dampen GLaDOS's power; to be as much of a moron as possible. He end up throwing Chell and GLaDOS down to the original Laboratories. He is finally stopped by being sent through a portal to the moon.
Mr. Johnson is the founder of Aperture Laboratories. He founded the facility sometime around 1950. His voice appears over the intercom in the old labs. As Chell progresses in the condemned sections of Aperture, he gets an illness he calls "moon-rock poisoning." Apparently, moon rock makes a good portal conductor, but is also toxic. After his death, GLaDOS takes over.
Caroline was Cave Johnson's secretary. She does not appear much in the game, but there are a handful of portraits of her scattered aroung the old facilities. While exploring the old labs, Chell learns that Caroline was uploaded to a computer and became GLaDOS.
Rat Man is a mysterious entity in the corners of the test chambers. He never appears in the game, but Chell finds much of his graffitti on the walls. He gives Chell advice, the most famous being "The cake is a lie," after GLaDOS promising a cake for Chell. The cake ends up being a trap. The picture is of some of his graffitti.