David is very close with his cousin Rosalind. She is not a direct full cousin. She is the daughter of his half-uncle Angus. Even though they are half-cousins they are very close because they share a secret: they can talk to each other in “thought-pictures” telepathically.
It must have been a self-preserving instinct which had made us keep the thing to ourselves, for we'd no active feeling of danger … (ch 4)
David, Rosalind, and the other telepaths are all deeply connected because they can talk to each other at any time about anything, and as David points out, “You can't lie when you talk with your thoughts” (ch 6).
When Petra falls in the river and uses her mind to strongly call David and Rosalind, they realize that she is not only one them, but the strongest one. Unfortunately, she is also the youngest (“a little over six” ch. 9). From then on David and Rosalind work together to keep Petra safe.
That fear is nothing compared to what they face when they are captured by the Fringe people and David’s uncle. Healthy women are rare, and they want Rosalind to help them make children.
His eyes travelled up and down Rosalind's slim straight figure in a way I did not care for -- nor she, either. Then he studied me more carefully, and nodded to himself, as if satisfied over something. (ch 14)
David’s uncle tells him that most of the women in the Fringes are hideous, and can’t have children because something is done to them to make them sterile before they are expelled. Rosalind sees his attention on her, and is terrified. David tries to jump them to save her, but they attack him and kick him out.
Rosalind is still trapped, but David is able to communicate with her. He loves her and worries for her, and Michael seems to be the one who is keeping everyone in control.
' I'm frightened of him. He's a different kind. Not like us. Not the same sort at all. It would be outrageous -- like an animal. I couldn't, ever ... If he tries to take me I shall kill myself. . . .' (ch 15)
David is able to maintain his strength again and help Rosalind. In the end, they are all rescued by the Sealand people.