Jim Hopper’s biological daughter, Sara, died from cancer in “Stranger Things.” Her death is a central part of Hopper’s backstory and explains much of his grief, guilt, and protectiveness.
The series reveals Sara’s illness through flashbacks showing Hopper and his former wife, Diane, caring for her during treatment. Sara eventually dies as a child, before the main events in Hawkins begin. Hopper carries her memory into his work and struggles to connect with other children because her loss remains emotionally raw.
Sara’s blue hair tie becomes an important symbol in the story. Hopper keeps it after her death, and it later connects him with Eleven, who wears it as their bond grows. The show occasionally uses poetic language about Hopper’s guilt “getting” Sara, but the illness presented in the flashbacks is cancer, not a supernatural disappearance or accident.