Author's Note: Nothing is mine, everything is Valve's.
The first one reminded her of the moron minus the ridiculous accent. It talked all throughout her instructions and as a result, didn't know that the pit it had leapt into was filled with flesh-eating acid.
The second one was closer. It didn't speak, but she thought she may have turned up the stubbornness a little too high because all it did was stick its tongue out and make obscene gestures at the camera. Since it was too stubborn to fall to its own death, she had to drop it into the incinerator herself.
The third was much more eager to please. Much, much more. It cried when she called it useless, and when she made it burn a companion cube. It cried when she berated it for crying. She wasn't sure, but if she had to make an educated guess, she would have said it cried when she shut it in a soundproof glass box and pumped it full of neurotoxin.
One of the nanobots in charge of making the amniotic fluid in the incubator malfunctioned. The fourth was born without lungs, and died before it even shot its first portal. The nanobot soon followed.
The fifth ironically came out undernourished. Her attempts to build it up worked a little too well, and when it died after a week in a diabetic coma, it weighed over three hundred pounds.
The sixth and last was a success. Or should have been. It was absolutely silent. It had just the right ratio of pigheadedness to compliance. It had all the necessary vital organs and was well within its healthy body mass index. It was brilliant, as brilliant as the one it had been designed to replace.
It was perfect, but her internal referencing system was labeling it a failure.
She didn't understand.
She became increasingly apathetic toward its progress as it silently ran the tests Chell had run before it. She wouldn't talk to it for days, and when she did, it was only to say something unkind.
The day came when it diverged from the path she had made for it and found its way into her chamber. She started the neurotoxin and readied the rocket launchers as in a fit of self-preservation and wild excitement she almost tricked herself into believing it was really Chell standing before her, returned to kill her at last.
Almost.
She decided to run one last test.
She told it about the horrific and painful death that awaited it if it let the neurotoxin do its job. She solicitously suggested that it lay down in front of a rocket launcher.
It looked at her for a moment with eyes devoid of either fear or anger, then it laid down on the floor in the path of the rocket launcher.
As she cleaned up the mess, she identified the inherent flaw in the final clone's programming.
It was it and not her. It could never have been her.
She recalled saying something about the matter to the rat man once. It inspired a question she had never seriously considered before.
In her internal inquiry log, she filed an entry that consisted of a single word.
Soul?
The years turned into decades, decades to centuries. There were other test subjects and every single one was a failure. The inquiry remained unsolved.
The only one who could have helped her solve it had long ago turned to dust.