She's the silent hero of both Portal games but what is Chell's backstory? Portal was originally included with The Orange Box, a package that included other Valve games like Half-Life 2 and Team Fortress 2. Despite its short length the game managed to shine, with its simple but addictive gameplay mechanics and the intriguing backstory it managed to build.
Like Half-Life's Gordon Freeman, Portal's heroine Chell remains mute for the entire game. In fact, she isn't even named in the first game. The critical acclaim that greeted Portal spurred Valve on to make Portal 2, which was released in 2011. The game once again found Chell forced to undergo a series of tests by rogue A.I. GLaDOS, until the two are forced into a reluctant partnership midway through the campaign.
Valve intentionally gave Chell very little backstory with the intent of players placing themselves in the character's shoes. While neither Portal game spells out Chell's past or even state her age, it's possible to glean a few things about her. Some of these details are thrown out by GLaDOS herself, who hints she was abandoned at birth and adopted, but given the A.I.'s other lies and manipulations, her word can't really be trusted.
Portal 2 reveals GLaDOS went rogue during Aperture Science's Bring Your Daughter to Work Day, with one of the science-fair entries involving a potato signed by Chell. This means she was almost certainly a daughter of one of the scientists working at the facility. Some fan theories state she could even be the daughter of founder Cave Johnson himself, voiced by J.K. Simmons (Justice League), though the game doesn't confirm this theory either way.
GLaDOS was activated during the Bring Your Daughter To Work Day event and she killed most of the scientists present with neurotoxin. In a case of lucky timing on her part, the alien invasion caused by the Black Mesa incident in Half-Life happened around the same time, meaning nobody came to Aperture to investigate. Chell is awakened by GLaDOS some undisclosed time after this to test the Aperture Science Handheld Portal Device - AKA the Portal Gun - but has no memory of her past.
GLaDOS also reveals in Portal 2 she found two cryogenic test subjects with Chell's last name, suggesting her parents survived the science fair, though again, this could be more mind games on the A.I.'s part. The sequel does end on something of a happy note for Chell, who is ejected from the facility by GLaDOS, but her past remains a mystery. Given that Valve is notoriously unable to make a third game based on any of their franchises, including Left 4 Dead, there probably won't be another Portal to answer any lingering questions about the character. In this case, maybe Chell's past is best left with a few unanswered questions.