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Although Chell's origins are unknown, she was definitely among the people present during GLaDOS' activation in 200-, as GLaDOS locked down the facility immediately afterward; ... According to the psychological profile in her personnel file, Chell is "abnormally stubborn" and refuses to give up, no matter how daunting the challenge. Due to this, she was rejected as a test subject, but Doug Rattmann altered the testing order, having correctly guessed that Chell's extreme tenacity might allow her to defeat GLaDOS.

Chell's Test Subject Application Form states that she refused to answer the essay question and instead answered in binary. The binary can be translated to "The cake is a lie."

She was not the fastest or most athletic of the test subjects GLaDOS captured (though she does appear to have strong stamina, strength, and balance), and some of the others had higher IQs, although Rattman implies that Chell's IQ was well above the average. Based on her accomplishments in the games, it can be surmised that Chell is highly resourceful, quick-thinking, very intelligent, strong-willed, good at creative problem-solving, and does not panic easily. Her file shows her fluency in binary code, demonstrating knowledge of coding. Chell's combined traits of athleticism, intelligence, and extreme tenacity make her an excellent test subject.

Chell never speaks to GLaDOS; in the first game, GLaDOS asks "Are you even listening to me?" and in the second, she calls Chell a "dangerous, mute lunatic."

[From the Half-Life Wiki.]

Chell is an athletic woman, about 5'4", in her 20s with dark brown hair that she generally wears pulled back. She is able to hear and make some sounds, but does not talk. Presumably in a pinch she can communicate in a textual medium, but whether she chooses to is another matter.

Chell is friendly enough if you're friendly to her; if you're a defective turret, a Companion Cube, a helpful core, she'll do what she can to help you out. If you try to kill her she will have few qualms about defending herself, though.

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Quick note on languages and formatting: Depending on the setting, Chell uses a combination of texting, writing, or sign language:
Courier for texting
Cursive/Sans for hand-writing
Italics for sign language.

Chell's signing is generally based on American Sign Language, but having been in stasis for a couple decades and only having robots with no hands to talk with, her approach to signing won't reflect what are best practices in the real world. If you're interested in ASL, the mun recommends you check out Handspeak, Lifeprint, and SignSchool for beginning resources!