Who is Rosemary?

Rosemary is a sixteen-year-old girl. She enters the story when David (the protagonist) crosses paths with her at his old family home.
David is initially shocked to find someone living at the abandoned house. It has been many years since anyone has lived there. When he enters the house, Rosemary isn't home. Depressed and heartsick, David lies down on a hard bed and wraps some quilts around himself. When he wakes up, Rosemary is cooking some eggs.
David tries to get up but soon realizes that Rosemary has tied his hands to the bedposts. Alarmed, he tells Rosemary that she is trespassing and that the house belongs to him. Despite their shared distrust, both eventually begin a conversation of sorts.
As David and Rosemary talk, they discover that they have both endured much pain in life. For her part, Rosemary has run away from her family; she is pregnant and must hide out until after her baby is born. Meanwhile, David confesses that he has kept a terrible secret from his wife for years. After discovering that his newborn daughter had Down syndrome many years ago, David gave the baby away. Although he thought it would spare his wife, Norah, from a lifetime of pain, his actions ended up destroying his marriage and family life.
David eventually returns home with Rosemary. There, Norah is upset to find that her husband has brought home a young, pregnant girl. She is not altogether convinced that David isn't having an affair with Rosemary. For his part, David denies that he had anything to do with Rosemary's condition: he only brought her home because he couldn't bring himself to leave her at the abandoned house.
Norah and David eventually separate. For his part, David purchases a duplex. He lives in one apartment while Rosemary lives in the other with her son, Jack. The two do not share a romantic relationship, but they enjoy each other's company. David spends time with Jack. And even though he cherishes those moments, he can't help but remember his own lost years with Paul, his son.
Eventually, Rosemary leaves; she is getting married and has reconciled with her mother and sister. So, she is moving to Harrisburg. David is sad that Rosemary is leaving, but he tells her that he understands. Before they part, Rosemary thanks David for everything he has done for her and Jack. In her final goodbye, she begs David to tell Norah and Paul the truth about Phoebe, the baby he gave away years ago.