Which characters were mother-like to David in the Chrysalids?

Sophie's mother was a mother-figure in David's life, and the fact that he would rather live with their family than his own is illustrated in the fact that he asks them if he can go with them when they flee.

David's aunt is a mother-figure to him as well. When she comes to visit her sister, he says that she has the features of his mother, but softened... the way that he wishes his mother to be. He sees that she cares about her child despite a defect, and he suspects that his own mother will not.

Rosalind is sort of a mother-figure in that she tries to take care of him and wakes him up to save his life, in the place of his own mother who should be doing such things.

Sophie could also be called a mother-figure because she saves his life and selflessly gives him up to save him.