David and Sophie were initially introduced in the first chapter of "The Chrysalids". David was sliding down a bank, and Sophie saw that he was having fun and decided to join him. David had found out a secret that Sophie and her parents were hiding, ever since she was born. The secret that David had promised to her to not tell anyone was that Sophie had six toes on each foot, which made her a deviation to mankind, and to the image of god. David wasn't bothered, or startled by the deviation, he insisted on seeing Sophie again later on, when she was ok. David had said "May I come and see Sophie again soon?" (pg.15), from that point on the two became really good friends. This is how their friendship had started and had lasted, until the end.
David and Rosalind are cousins, which made their friendship even stronger. David and Rosalind both have a deviation of their own, that no one knew, at first. The deviation they had was they were able to communicate telepathically. They first realized this when David was talking to someone and he found out it was Rosalind. They got to know each other better, and became good cousins, but better friends. "Rosalind rested her hand on my arm." (pg.181) Having this deviation and being cousins made it really hard for them to not hate each other, which made them friends for a long