Although Chell's origins are unknown, she was most definitely among the people present during GLaDOS's activation, as GLaDOS locked down the facility after her activation, letting none escape, or be let in from the outside.
Based on her success in the Test Chambers and subsequent escape, it can be surmised that Chell is highly determined and a quick, inventive thinker. Other than that, little is known about her personality and background. All further information about her comes from comments by GLaDOS, who is by no means a reliable source of information, since she seems to often lie to Chell to make her feel guilty about her actions. For instance, according to GLaDOS, Chell's personnel file says she was adopted and is "unlikable" in character.
Chell was in the Aperture Laboratories at the time of GLaDOS' takeover, meaning she had some business there. This is supported when GLaDOS angrily tells her "You're not smart. You're not a scientist. You're not a doctor. You're not even a full-time employee!", suggesting she had some sort of part-time job at Aperture. However, frequent mention of the Aperture Science first-annual-bring-your-daughter-to-work-day, which occurred on the day of GLaDOS' takeover, suggests that Chell was an employees' daughter, who was captured for GLaDOS' permanent testing cycle. Unless, of course, GLaDOS is lying again.In an apparent attempt to convince Chell not to kill her, GLaDOS tells her, "I have your brain scanned and permanently backed up in case something terrible happens to you." Later, in a fit of anger, she claims to have deleted this backup, adding, "The part of you that could have survived indefinitely is gone. I just struck you from the permanent record. Your entire life has been a mathematical error... a mathematical error I'm about to correct." It is currently unknown why GLaDOS might have done this (fan speculation often points at Chell being a clone or an android), and whether it might be a hint about Chell's origins. On the other hand, the whole idea might well be a complete fabrication, like many of GLaDOS' other statements.
As a Test Subject, Chell also is fitted with Advanced Knee Replacement, small spring devices surgically attached to her legs that assist her jumping and walking skills and allow her to fall great distances without being hurt.
Chell appears to be in her late 20s or early 30s, and of mixed ancestry or possibly Latin or Japanese descent (Chell's face and body model, Alésia Glidewell, is of Brazilian and Japanese ancestry), and is rather thin. Throughout the game she wears a worn-out (especially around the knees) orange jumpsuit and has bare feet. She has a ponytail and mild "bed hair" (with streaks of grey, mostly on the left side of her head) from sleeping in a stasis pod for an unknown period of time, dark and tired eyes, and Advanced Knee Replacement prosthesis surgically attached to her legs. It also seems that she used to wear earrings that were apparently torn off.