Rachel Carlson was Juliet's sister.
A young Rachel is upset upon hearing the news of her parents' divorce in ("The Incident, Part 2").
Later in life, Rachel developed cancer, and her chemotherapy treatments left her sterile. Juliet secretly treated her with drugs Juliet had stolen from Edmund Burke's laboratory. In 2001, Rachel revealed that she was miraculously pregnant, indicating that the medications Juliet gave her worked. ("Not in Portland")
Juliet took a job at Mittelos Bioscience, and Rachel accompanied Juliet to a small airport near Miami, where they were met by Ethan Rom and Richard Alpert. Juliet expressed concern about being away from Rachel, but Rachel reminded her that the cancer had gone into remission. Rachel and Juliet tearfully said goodbye, and Juliet left. ("One of Us")
Rachel with her son Julian.
Rachel's cancer returned, according to documents Ben Linus showed Juliet. Ben said that Rachel would die a few months. However, he also swore that if Juliet stayed on the Island, he would cure Rachel. When Juliet replied that Ben would never bring Rachel to the Island, he replied that Jacob would take care of it himself. Juliet did stay, and on Wednesday, September 22, 2004, Ben took Juliet to the Flame to see a live video feed of a healthy Rachel playing on a playground in Acadia Park with her son, Julian. ("One of Us")
- In her room, Rachel kept an octagon with the yin / yang symbol in its center. This is part of the pattern of the bagua which inspired the DHARMA logos.
- Additionally, there was a copy of Carrie on the same bedstand.
- Rachel had dark hair as a child, but had blonde hair on the video Ben showed to Juliet.
- Rachel is the name of Jacob's wife in the Book of Genesis. Rachel was disgraced because she had borne no children to Jacob, but her sister and second wife of Jacob, Leah, bore six children (Genesis 30). In the end, God grants Rachel children, one of whom is Joseph (essentially restoring her honor). Rachel died in childbirth with her second son Benjamin.
- Her last name appeared on the medical chart that Ben showed Juliet revealing that Rachel's cancer had returned. ("One of Us")
- The name "Rachel Carlson" is similar to Rachel Carson, who is the author of "Silent Spring", a seminal work that documented the harmful effects of pesticides on birds and on the environment as a whole. Carson's research showed that pesticides like DDT caused the egg shells of birds to become too thin, among other reproductive problems.
Savannah Lathem depicted the young Rachel in "The Incident, Part 1". The casting call described her as "Young Donna, Caucasian. 12-year-old brunette. Natalie's older sister. Tries to act more grown-up and stoic. Can hide her feelings while Natalie acts out. Her parents tell her they are divorcing, and Donna is upset, quietly crying." 
- Did her cancer actually come back or was Ben lying?
- Did Jacob heal her as Ben claimed?