The character of Sophie in this text is used to indicate to the reader the fate that mutants face in this dystopian world of the future. It is key to realise that Sophie is introduced as a child to our narrator, whose eyes we see his world through. David thinks there is nothing wrong with an extra toe, and from his child-like perspective, chooses to overlook that rather than let it interfere with his friendship with Sophie. However, the brutality with which Sophie is hunted down and caught, even though she is nothing but a child, clearly indicates the cold, chilling brutality of the society that David is growing up in, and serves both to foreshadow the way that David and Petra will be hunted down and also represent the danger of what could happen to them if they are caught.
Sophie is reintroduced into the novel towards the end as an adult to highlight this particular aspect of the novel. Note what she says about her life and her relationship with Gordon and how this engages the reader's sympathy for her:
You've never known loneliness. You can't understand the awful emptiness that's waiting all round us here. I'd have given him babies gladly, if I could. I--oh, why do they do that to us? Why didn't they kill me? It would have been kinder than this...
The emotion is palpable in this quote as Sophie confronts her inability to have children because of her forced sterilisation and the loneliness of her life. Sophie is made to be such a tragic figure therefore to highlight the cruel, calculating system of governance that is determined to eradicate any sign of mutation, and also to foreshadow the zeal by which this same system will hunt down our hero and his fellow telepaths later on in the novel.