I have been looking around the web for an answer but I can't really find anything. How did the character in the video game Portal end up at the lab, trapped by GLaDOS?
It is clear from answers to How was Chell picked? and info on Wikipedia that she was removed from the test subject queue by a former Aperture employee who moved her to the top of it, however it doesn't explain why she was at Aperture labs in the first place.
Does anyone know what really happened before the actual game starts?
Chell's backstory has to be inferred from content in the game, as very little is made explicit. We know from Valve's pre Portal 2 Comic "Lab Rat" that Doug Ratmann placed her at the top of the testing queue because GladOS had previously rejected her as being unsuitable for testing due to excessive tenacity, but this tells us little about her origins.
Portal 2 spoilers follow:
One of the science exhibits at the "Bring your daughter to work" day is indicated as being by Chell. While everyone else seems to have made a potato clock, Chell created a mutant potato with accelerated growth. From this we can conclude (a) Chell is the daughter of an Aperture worker (b) she appears to be something of a scientific genius and (c) she has apparently aged somewhat by the time of the original Portal. (This aspect of the game is somewhat problematic - how has she had time to grow to adulthood if only woken for periodic tests? One can assume she's been in cryo for quite some time, but that doesn't seem consistent with Ratmann still being alive.)
More spoilers, including speculation based on in-game content.
GladOS finally letting go of Chell at the end of Portal 2 offers some clue as to Chell's parentage. As revealed by Portal 2 the personality upon which GladOS is based is that of Cave Johnson's secretary Caroline. Although GladOS claims to have erased Caroline at the end of Portal 2, it wouldn't be the first time she has lied. The Turret Opera in particular seems to be a song expressing deep motherly affection for Chell. It is likely that Chell's mother is actually Caroline. Her father remains a matter of pure speculation, but given Cave Johnson's apparent fondness for Caroline (as exhibited in the fact he wanted her to be used as the AI template), the idea that Cave Johnson himself is Chell's father remains a strong possibility. This is possibly backed up by Chell's scientific prowess, but it's never clear whether Cave Johnson is responsible for any of the actual Science at Apperture or whether he is a non-scientist CEO.
More background on Chell can be found here.
Chell's backstory is actually fairly inconsistent. The science fair project implies that she was there as the daughter of a scientist during bring your daughter to work day, but she'd probably be fairly young at that point in time. My guess is ten or eleven, possibly even younger. However, it is shown in the comic Lab Rat that Chell signs up to be a volunteer test subject but is denied. I'm pretty sure she'd have to be eighteen to legally do that, although Aperture isn't really known for following the law to the tee. She'd also still have to be at Aperture on that specific day to be trapped. This gives two different supposed ages for Chell being stuck in Aperture. I'm going to go with a bit above eighteen because it's the only thing that makes sense given the whole Rattmann storyline going on, too, alongside Portal. Unless GLaDOS has some weird aging serum. I wouldn't put it past her to gain a testable subject through scientific means rather than natural.
I would say that the work of Chell at the "Bring your daughter to work day" is very old. She had plenty of time to grow old between the time of this work and the time where Aperture Science became more elaborated with the new test rooms and the creation of Glados.
I guess she applied around 20-30 years old to be a test subject. Probably because employees of Aperture Science are asked to be test subjects themselves, as said by Cave Johnson in the game. So she may have this motivation from her parents, who seems to be working at Aperture Science (maybe Caroline and Cave Johnson, but we don't know).
Then, she was trapped with GLaDOS while at Aperture Science. Maybe she was in a relaxation chamber, testing whether she could be a test subject, or whatever (we don't know).
I think it makes sense like this regarding her age.
GLaDOS says in Portal 2 that she was abandoned on a doorstep so maybe they found her as a baby and kept her as a test subject.
