Chell: An Aperture Science human female test subject. Any information regarding her background, age, or even how she became a test subject all remain a mystery. What is known is that she must complete 19 Enrichment Center test chambers per GLaDOS (Genetic Lifeform and Disk Operating System), an AI developed by Aperture. Her only equipment provided in accomplishing these chambers are the Aperture Science Handheld Portal Device or Portal Gun for short, and heel springs, which allow her to fall great distances without the unfortunate side effect of death.

While traversing the Enrichment Center, Chell encounters no other human test subjects. Save for GLaDOS, several robotic gun turrets, and the ill-fated companion cube, the entire facility appears to be completely abandoned.

GLaDOS: Originally designed as an ice inhibitor by Aperture Science in order to compete with Black Mesa for a government contract. Aperture decides to not only make an ice inhibitor but also a Genetic Lifeform and Disk Operating System. GLaDOS is in development for more than 10 years before finally becoming fully operational.
    "She" later becomes Chell's guide throughout the various test chambers at the Enrichment Center, providing information about the Portal Gun and certain aspects of Aperture Science. "Her" goal in doing this is seemingly to get Chell to complete all 19 test chambers. Upon which "she" has promised cake and grief counseling.

The Weighted Companion Cube: A metallic cube with a heart. Whether or not this metallic cube is able to speak is up for debate. But despite this debate, the Companion Cube is at it's very heart (if it has one): A useful stepping stone, a shield against those that would do the test subjects harm, and a weighted force that can open doors for the test subjects. The Companion Cube is truly the best friend any test subject at the Enrichment Center could ask for.