(I couldn't be arsed to translate in to English myself, so I used google translate)

Here are my opinions on some conspiracy theories about who / what is Chell.

1. Chell can not possibly be a robot / android, this was precisely what GLaDOS replaced her with.

2. Gordon Freeman is a scientist, Chell is more subtle. Both are intelligent, both shut up even though according to Valve, they can "talk". And both live at the same time. There is nothing obvious that backs up the theory that Gordon and Chell are siblings, or at all know each other.

3. Chell never gives up, as shown in the "Lab Rat", literally - she is tested. You can back up the theory that she is a clone. She should not really be tested until a few years later. She was number 1498, Doug Ratman changed this to # 1, as she was the next to wake up and be tested. Ratman believed that she had the personality of that needed to destroy GLaDOS. Which she did, twice! But something that is doubtful whether this clone theory is that she was so unwelcome, why make clones of her?

4. Chell aka Michelle Johnson - correct, Michelle Johnson, daughter of Cave and Caroline Johnson. Something that backs up this theory very strongly, is that Chell being bullied by both GLaDOS and Wheatley that she "adopted". You can also see Chells signature on one of the "Take your daugher to work day" signs in Portal 2 In the elephantine potato tree. You can read that she refers to Mr. Johnson and his father differently. Her father is forced to be a researcher in Aperture Laboratories, and it is quite obvious that Cave has forced one of its employees to adopt Chell after birth. So Johnson pair did not have to take care of Chell, and still be close to them. You can also read that Chell, actually behind the potato. "Take your daughter to work" day all know was the day GLaDOS woke up and started killing everyone with nerve gas (Neurotoxin). It is believed that during all that, people were put into an induced coma. And that may be how Chell was a test person. Back to "Take your daughter to work" poster I talked about earlier. You can see that the letters "n", "e" and "u" is missing - so it stands ergy E of the F Overture - these letters are the first three letters of the word Neurotoxin (nerve gas).

5. At the end of Portal 2, you'll see these turrets come, sing opera. In the game it is sung in Latin, but you translate it to English you can hear a very suspicious text:
"Beautiful Dear, My Darling Beauty.
My baby, oh heaven. ("Heaven" means "Ciel" in Latin, as the name "Chell" comes from)
That she (GLaDOS) esteems.
Oh My Dear, farewell.
My Dear Child.
Why do not you walk far away.
Yes, far away from science.
My dear, dear baby.
Ah, my beauty.
Ah, my dear.
Ah, my little girl.
Oh, my dearest one. "
As everyone knows, GLaDOS actually Caroline - Cave Johnson talked about putting minds on the CD, which he most likely did with GLaDOS when Caroline died - and the beginning of Portal 2, GLaDOS is full of hatred, but when she realizes that she once was Caroline, she expresses in the song "My Beauty", "My Dear," "My little girl", and "My Baby". GLaDOS also found out that she is Chells mother. She could have killed Chell a bunch of times at the end of the games. One of the most obvious places, will probably be right before the turret opera. Where these four turret aiming at Chell. Ikka just let GLaDOS Chell survive, she let her go from Aperture Science. The turret opera sing GLaDOS "Why do not you walk away, far away from science?". "Science" more specifically = Aperture Science.

6. I should have been done to explain now, but I really want to guess at one thing: Who is Chells adoptive father?
We know that the adoptive father works for Aperture, and that he is a scientist. One thinks of the people who are being introduced in the games, there are really only two choices; No. 1 is that it is a scientist we do not know yet. Number 2 is that it is Doug Ratman. Doug shows special care for Chell. It can be simply just hoped that she can destroy GLaDOS, but it could also be that he has a father-daughter relationship with her. We know that Doug knew about Chell and her personality before she became a test person. There would be no doubt that Doug Ratman is her adoptive father.