The Ghost of Redemption by Aron Lewes

Nicholas is a profoundly lonely man, mourning the loss of a dead wife and a daughter who no longer loves him. His best and only friend is Lillie, a spirit who is always at his side. They do everything together, he just can’t see her.
Lillie has been Nicholas’ spirit guide for a long time, and knows everything about him. She understands his pain, forgives his mistakes, and knows his wishes… and it’s her job to make them come true.

This novella is a spinoff of the YA series School for Spirits, and can be read as a standalone story.