Chell\'s history is unclear, as GLaDOS is the only source of information and she is an unreliable source.
Chell may have been abandoned by her birth parents and may have been adopted. Or, she may be an employee\'s daughter, who was present at the 1998 Bring Your Daughter to Work Day (one of the science fair projects in Portal 2 is signed \'by Chell\').
We know that Chell has been in the Aperture Science Enrichment Center\'s Relaxation Vault for a very long time. She is mute, possibly as a result of brain damage from excess Relaxation (or neurotoxin, whatever).
In Portal 1, Chell is equipped with Advanced Knee Replacements, which allow her to fall long distances without injury. They are replaced by the Long Fall Boots in Portal 2 (in both cases, Chell wakes up with the equipment already installed).

EL 13: Portal Series