Sophie Wender in the Chrysalids

Sophie is David's friend, which is significant because she is someone that his father would reject, and partially how he learns to think about mutations as being okay. Her discovery, and the capture of her family made an impact on David's childhood.

Later, when he meets her again, she has been abused and sterilized, but she still loves him. He has led a more privileged life and has moved on, and meeting again is awkward and probably hard for both parties, but obviously so for Sophie.

She is killed in the final battle, as is David's father and the Spider Man, thus providing an easy out for the Zealander's genocide... no one is left that David knew well or loved.