Sophie is an outcast because she has six toes.  In her society, anyone who is different is taken from the family and expelled to the Fringes to live among heathens.  Her family protects her by keeping her hidden for most of her early childhood.

Being an outcast has kept Sophie very isolated, and therefore timid.  She is not used to interacting with people, and even though she is a gentle and warm soul she has no friends because she is not supposed to exist.

When David first sees Sophie, he is shocked because he knows all the kids for miles around.

She stood still for a few seconds as though uncertain about leaving the security of the bushes, then curiosity got the better of her caution, and she stepped out. (ch 1)

Sophie befriends David, and appreciates his help.  She is young and in an impossible situation.  She gets her foot caught and David has to help her out.  He promises her parents that he won’t tell, and despite the fact that he is Joseph Strorm’s son they believe him.

A blasphemy was, as had been impressed upon me often enough, a frightful thing. Yet there was nothing frightful about Sophie.  (ch 1)

Unfortunately, David cannot protect Sophie forever.  When a man named Alan sees Sophie’s footprint, the game is up.  Sophie is sterilized and expelled.  Before she goes, she gives David a lock of hair and a “piece of yellow ribbon” and kisses him (ch 5).

When David sees Sophie again, he barely recognizes her.  Her youthful innocence gone, she is now the mistress of David’s Uncle on the Fringes.  She cannot give him children, and worries that he is going to replace her with Rosalind.  She is curious about David, but one an adult level now.

What impact does being an outcast have on shaping David's perspective in The Chrysalids?  

Since David is an outcast, he is more sensitive and sympathetic.  An example of this is when he tries to protect Sophie.

When David encounters Sophie, he is about ten years old.  He has never met anyone with a mutation before, and like most of his people he assumes that they are monsters.  When he meets the girl with six toes, he keeps her secret despite the fact that his father is the second generation founder of Waknuk.  Waknuk is extremely dictatorial, with anyone who does not fit into their narrow view of the world expelled.

 I tried to explain that a person with a deviation -- a small deviation, at any rate -- wasn't the monstrosity we had been told. (ch 6)

David is also different from others because he is telepathic.  So he realizes that Sophie needs to be protected, and he goes out of his way to try to keep her safe.

I ran up the stone and flung myself on him. He was bigger than I was, but it took him by surprise, and we went down together in a whirl of arms and legs. All I knew of fighting was what I had learnt from a few sharp scuffles. (ch 5)

David tries to fight Alan to protect Sophie because he can understand how she feels.  He knows she is just a normal little girl who got a bum rap from fate.  Without much of an interest in himself, he tries to fight a much bigger man just to buy her time.

David's deviation is much easier to hide than Sophie's, but he still understands how she feels.  He regrets betraying her, but he could not help it.  His father made sure of that.