In an article for Writer’s Digest, Barbara Josselsohn says ghosts can help you develop the characters in your story. Examples are abundant, from Noel Coward’s Blithe Spirit to the Oscar-winning Ghost. “Other-worldly characters are often the means by which we learn about a human character’s unresolved questions, fears, and sadness,” Josselsohn says.
How exactly does this help? Ghosts – or aliens or time travelers – can help us explain our characters. “Sometimes we’re way too close to a character to truly reveal them to readers,” Josselsohn notes. “Their behaviors or reactions are second nature to us, because they are our behaviors or reactions—but they can be confusing to readers who don’t know the characters as well as we do.”
As an exercise, Josselsohn suggests adding an otherwordly character to a scene. What does that being observe? What questions do they have? How can you use those insights to reveal your protagonist? You might give those observations to another character in your novel or even decide to keep the ghost.