The specific backstory event that emotionally damaged your character and set the stage for their flaws, fears, and false beliefs.
The character ghost is the concrete event or experience in a character's past that created their emotional wound. While the wound is the ongoing emotional state (like a fear of abandonment), the ghost is the actual thing that happened (like a parent leaving on their tenth birthday). The ghost typically occurs before the story begins and haunts the character throughout - hence the name. It doesn't have to be a single dramatic event; it can be a pattern of experiences, but the best ghosts are specific enough that you could write the scene.
A ghost gives your character's psychology a solid foundation. Instead of vaguely knowing your character is 'damaged,' you know exactly what happened, when, and why it still echoes. This specificity makes your character's behavior more convincing and your story more emotionally coherent. It also gives you material for powerful reveals and flashback moments when the time is right.
Carl's ghost is Ellie's death and their unfulfilled dream of visiting Paradise Falls. That opening montage is essentially a ghost sequence that defines every choice Carl makes.
Harry's ghost is the murder of his parents by Voldemort - an event he can't remember but that shapes his identity, his relationships, and his sense of being marked as different.
Jude's ghost is the extensive abuse he suffered as a child, which the novel gradually reveals in devastating layers over hundreds of pages.
Each Crain sibling carries a different ghost from their night at Hill House, and the show structures entire episodes around revealing how those specific events warped each character.
Be specific. Don't say 'a difficult childhood.' Say 'the afternoon they came home and found every photo of them removed from the walls.'
Show the effects of the ghost first. Let readers wonder why the character acts this way before you show them the cause.
Trace the chain: ghost causes wound, wound creates lie, lie drives flaw, flaw produces conflict. Every link should feel inevitable.
Write the ghost scene for your protagonist - the specific moment that changed everything. Keep it under 300 words. Focus on one sensory detail (a sound, a smell, a texture) that your character would still remember years later. Then write a brief present-day scene where that same sensory detail appears and watch how your character reacts.