Chell

Chell (last name unknown) is the silent protagonist of Portal. She appears to be in her late 20’s or early 30’s, and possibly of Latina descent (Chell’s face and body model, Alesia Glidewell, is of Brazilian and Japanese ancestry). Throughout the game she wears an orange jumpsuit and is barefoot, with heel springs attached to her legs.

Gameplay abilities
Chell cannot be injured by falls of any distance. The game developers added the heel springs to explain this.

She also regenerates health rapidly; although she can be killed by a few seconds of being shot by a gun turret, if she manages to retreat to safety first she quickly returns to full health. However, so far this seems to be simply a gameplay convention and not a story element.

Role in Portal
At the beginning of the game, Chell is imprisoned in a doorless “Relaxation Vault” at the Aperture Science Enrichment Center. GLaDOS opens a portal in the vault and directs her through a series of increasingly dangerous “tests” involving the Aperture Science Handheld Portal Device, promising cake as a reward at the end of the experiments.

Upon completing the final test chamber, Chell finds herself about to be plunged into a furnace. Although apparently not intended to survive, she quickly escapes to safety through clever use of the ASHPD. From this point forward, she navigates through abandoned portions of the Enrichment Center with the help of portals, her own ingenuity, and graffiti messages apparently left by a former escaped test subject.

Eventually making her way to the central GLaDOS unit, she dislodges and incinerates four of GLaDOS’ personality cores, triggering a large explosion. The song “Still Alive,” which GLaDOS sings during the credits, implies that Chell and GLaDOS survived the explosion and Chell escaped into the outside world.

Character
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.

Upon completion of a test chamber, GLaDOS often praises Chell’s resolve and intelligence, although these may be scripted responses (she only addresses Chell as “Subject Name Here”). She claims that Chell euthanized her Weighted Companion Cube faster than any other test subject, an accomplishment GLaDOS seems to feel is cause for congratulation.

After Chell escapes, GLaDOS grows increasingly hostile, informing Chell that she is “not a good person” and complaining in response to her silence, “you don’t even care, do you?” According to GLaDOS, Chell's personnel file says she was adopted and is "unlikable" in character.

In contrast to her earlier praise, during the final boss fight GLaDOS says, “You’re not smart. You’re not a scientist. You’re not a doctor. You’re not even a full-time employee,” which seems to imply (but does not necessarily entail, even if GLaDOS is telling the truth) that Chell held some sort of part-time job at Aperture before GLaDOS’ takeover. Alternately, it could be possible that GLaDOS is spitefully lying to you once again, and you really were a doctor and/or scientist and/or full-time employee, who either willingly volunteered for testing in the Enrichment Center or was forced into it by higher authorities. Also, numerous references to the "Aperture Science Take-Your-Daughter-to-Work Day" in the game and in the related viral website imply that this may be a possibility for the method Chell came into the facility.

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, 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.

These backups, if true, may explain the evidence of other test subjects in the facility. Chell could have possibily been restored from backup in the "relaxation pod" provided for her, in essense being the person who left the messages and warrens in the facility. If so, that means that Chell (or someone else) escaped the reclaimation process in the past.

It is also possible that this might reference to the in-game ability to start the test over from the beginning after death, as sort of a pseudo-explanation for the unrealistic gameplay element. This is also referenced to in Half-Life.