Sara Patrell is a girl that Will Gorski met as a child, and a sensate. She died when Will was a young boy, and became an unsolved missing-person case. As a cop, Will obsessed over Sara's case file, trying to understand what happened. As Will's powers gradually awaken, he eventually recalls the truth: Sara was captured and lobotomized by Whispers.
Sara and Will experienced a limited sensate connection as children. Will interpreted the vision of her as seeing a ghost. In one episode, young Will is seen talking to a therapist, who tries in vain to convince Will that he's not seeing her. (I Can't Leave Her)
The show does not directly show Sara's death, but J. Michael Straczynski has independently confirmed that Sara is deceased.