Chell

Chell is the silent protagonist of Portal. She appears to be in her late 20's or early 30's, and possibly of Latin descent (Chell's face and body model, Alesia Glidewell, is of Brazilian and Japanese ancestry, although the character Chell has blue eyes.) Throughout the game she wears an orange jumpsuit and is barefoot, has mild "bedhead" from sleeping in a stasis pod for an unknown period of time, 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' worth of turret gunfire, if she manages to retreat to safety she quickly returns to full health (this appears to be simply a gameplay convention and not a story element). If she is hit in the face by something, such as the Weighted Storage Cube, Chell actually grunts with pain.

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 and grief counseling 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 portal gun. 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?", and even saying towards the end: "That's it. I'm going to kill you, and all the cake is gone". 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. It could be possible that GLaDOS is spitefully lying to Chell once again, and she in fact really was one of those things previously, 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 possibly been restored from backup in the "relaxation pod" provided for her, in essence being the person who left the messages and warnings in the facility. If so, that means that Chell (or someone else) escaped the reclamation process in the past. However, since there are no graffiti in GLaDOS' chamber, it is entirely possible that Chell was the first to destroy GLaDOS and escape.

Behind the scenes
The heel springs were originally used for the Combine Assassin, an enemy cut from Half-Life 2.