Rachel is one of the secondary characters in The Chrysalids. Rachel is Anne’s sister and both girls are part of David’s circle of deviants. Since she is a minor character, she is not developed by Wyndham and little can be deduced about her character traits. Petra says "does seem to cry an awful lot, that girl," and Michael says she is brave and courageous. She loves her sister and is genuinely distressed at Anne's suffering and naturally shaken by her sister's final action. She is also sensible because she burns the note left behind by Anne.
We know more about how Rachel functions in the novel, which is largely to provide information. David explains things to her or she tells David about people or incidents, like Mark or Anne’s troubles. In this function of information bearer, Rachel has part in three big incidents. After Anne marries Alan and confesses her powers to him as well as naming the others, Rachel is the conduit for all the information about Anne's condition that David and the others receive after Alan is found in the field. Rachel is even asked by David to try to deliver information to Anne:
'You must get a note to her somehow, Rachel,' [David] added. 'Word it carefully so that she'll understand ... .'
'Very well. I'll try,' Rachel agreed doubtfully ....
Most importantly, on the morning she goes to comfort Anne, she is the one who, in company with Anne's neighbor, finds Anne lifeless in her bedroom. The note Anne left behind is assumed to be a suicide note and pressed into Rachel's hand. In fact, it is a list exposing all their names. Rachel's presence has saved them all from exposure (as did Uncle Axel's action).
The second incident Rachel is importantly involved in is the escape. Rachel did not escape with the others. As they are about to depart for Zealand, it is Petra who reaches her for one last message because Rachel is out of range for all but Petra. It is while Petra is communicating with Rachel that she sees Rachel's between-think feelings. Despite David's admonition against noticing between-think, Petra continually does so, thus suggesting through contact with Rachel the entrance of a new more powerful paradigm that includes noticing between-think. During the same incident, Rachel learns that Michael is declining to leave with the others for Zealand. He tells Rachel through Petra (who reads Rachel's "sort of happy-crying" between-think) that he will come to "fetch her out" (rescue her). Rachel, the information provider, provides the final subtext of heroism, courage, and hope.