In a nutshell, the hardships of her existence as somebody bearing a physical abnormality have transformed Sophie from an innocent child to a woman who has been toughened by life's hardships. After years of living like a "deviant" who has known incredible loneliness, she feels, frankly, that it would have been kinder for her to have been killed than to live the life that she has been living.

As a child, Sophie had been "normal" in spite of her constant need to hide her six toes. She was just a regular innocent and mischievous child, who probably wasn't yet aware of everything that being a Deviant would come to mean in her life.

By chapter 15, however, Sophie is a drastically changed woman. She is intent on survival, and will do whatever it takes. She is markedly more aggressive than she was as a child, and she now has full awareness of everything that her Deviant status entails. She has become as tough as nails thanks to her circumstances, and does not hesitate to take other people's lives when necessary.

The formerly innocent child has, as a direct result of being a Deviant and constantly having to watch her back, become a seemingly cold-blooded killer. She has also been rendered infertile, and has notable jealousy towards women who are able to bear children.

By the time David meets Sophie for the second time, her appearance and character have changed dramatically. In the first chapter, David describes Sophie as a typical happy child, still in awe with the world:

The branches parted, and a face looked out at me. It was a small face, sunburned, and clustered about by dark curls. The expression was somewhat serious, but the eyes sparkled.

In the fifteenth chapter, the spark in her eyes and her dark curls have been replaced by a "glint of dark eyes" and "dark hair hanging down on each side of her sunburnt face."

The once "ordinary little girl" has been worn down and made bitter by living on the fringes and being what she calls a "deviant."

You've never known loneliness. You can't understand the awful emptiness that's waiting all round us here . . . Why didn't they kill me? It would have been kinder than this.

Her only her solace is her relationship with Gordon, but even that is threatened by Gordon's interest in Rosalind. Even when David tries to convince her that Rosalind is not interested, she finds it very hard to believe him. It's almost as if she has nothing else left.

"You can't know a thing like that about another person. You're just trying to—"
"I'm not, Sophie. I do know. You and I could only know very little about one another. But with Rosalind it is different: it's part of what thinking-together means."
She regarded me doubtfully.
"Is that really true? I don't understand—"
"How should you? But it is true. I could feel what she was feeling about the spi—about the man."
She went on looking at me, a trifle uneasily.

Her change in character is represented most dramatically, however, when she leaves the cave to kill someone.

The broad blade was clean and bright. It looked as if it might one have been part of the kitchen furnishings of a raided farm. She slipped it into the belt of her skirt, leaving only the dark handle protruding.

When David and his friends flee to the Fringes, they are jumped on from the overhanging branches. The two girls with him are taken to become breeders since the Deviants cannot reproduce (they have been sterilized). After having been jumped on, David is also dragged outside the Fringe village and beaten severely. When he regains consciousness, he is being dragged by Sophie Wender, who has not forgotten her friend.

Sophie is on her own now and tells David that she will aid him in retrieving the two girls because she does not want Gordon, the extremely tall man who resembles David's father, to copulate with them. From her words and emotions, David gathers that Sophie loves Gordon:

"If she were to give him children, he wouldn't want me any more," she said at last.

After she and David enter the camp, Sophie goes into the tent where an albino stands guard over the girls. However, after a short time, the girls emerge, then Sophie exits, her arms covered with blood. Sophie has obviously become very hardened, and David regrets that he and Rosalind are responsible for Sophie's jealousy and her loss of the innocence that she demonstrated as a young girl who only feared that someone would notice her extra toes. Added to this, David worries for Sophie's safety, but he and the two girls must move on as the men from town pursue them. 

In the beginning of the novel, Sophie is basically a normal, good natured kid.  She enjoys spending time with David, and is a bit naive about the cultural implications of being a Deviant.  She is a bit more worldly than David, because she knows that she should hide the fact that she has six toes. That's probably because her genetic mutation is visible to her and her parents.  She has been living with it for her entire life.  At the same time David doesn't know about his abilities just yet, so it makes sense that he is more naive than her.  Still, she's not a hardened killer.  

By the time the reader finishes chapter 15, though, it is evident that Sophie is a vastly different person than she used to be.  Her change is not only focused on external aggression and survival, but also her internal attitude about life and society is very different.  I like this quote:

"To be any kind of deviant is to be hurt—always," she said.

It shows that Sophie is not disillusioned about being a Deviant any longer.  It hurt her to be ostracized as a child.  It was painful to be sterilized, and it continues to hurt because she is incapable of bearing children with/for the man she is in love with. That constant pain and suffering at the hands of an intolerant society has made her a cold and hard person.  Rosalind and Petra bear witness to that attitude when Sophie kills the guard with little to no hesitance.  The final lines of the chapter summarize nicely exactly how much Sophie has changed. 

"Rosalind, and Petra too, watched silently in horrid fascination as Sophie scooped a bowlful of water from the bucket to wash the blood off her arms and clean the knife."

She's cool, calm, and collected after having killed someone. That's a far cry from the girl readers first met.  

In Chapter 15, Sophie resurfaces in David's life. When he regains consciousness from his beating for having attacked one of the men in the Fringes for lewdly looking at Rosalind, he finds himself being dragged by Sophie Wender.

(1) She has matured, of course, and she is in love with Spider-Man, David's uncle, whose real name is Gordon.

(2) David learns that she is infertile, as the deviants have been rendered this way so that they would not multiply. Also, because she envies the girls who can have babies and wants to take them away from Gordon, she tells David that she will help him rescue the two girls that Gordon has taken as "breeders" so that he and others can have children. When they get back to the camp, Sophie goes in the tent and frees the girls, and when she emerges she has blood on her arms.

(3) She is capable, not only of envy, but of murdering the albino who guards Rosalind. David regrets that he and Rosalind are responsible for Sophie's loss of innocence and jealousy, and he worries for her safety, but they must move on as the men from town approach.