General terms:
* Spirit
* Phantom
* Specter
* Apparition
* Haunt
* Shade
* Wraith
* Ethereal being
* Unseen presence
* Otherworldly entity
More specific terms:
* Poltergeist: A mischievous or malevolent spirit
* Doppelganger: A ghostly double of a living person
* Revenant: A returned spirit, often seeking revenge
* Golem: A magically animated being, often made of clay
* Demon: An evil spirit
* Banshee: A female spirit who cries out to warn of death
Figurative terms:
* Shadow
* Echo
* Memory
* Past
* Residue
The best word to use will depend on the specific situation and the desired effect.