I'm not entirely sure without reading it more closely, but often in zombie novels there's a strong, resourceful protagonist. Maybe there's someone like that in this novel who leads the fight against the zombies.
Celestia, being a key figure in her own realm, would play a major role. The Reapers, as the mysterious and often feared entities, would also be central. Depending on the fanfic's take, there could be a character who acts as a mediator between Celestia and the Reapers, perhaps someone who has knowledge of both their worlds and tries to bridge the gap between them.
The Reapers have a back story that spans eons. Essentially, they are a product of a very old form of technology. Their origin lies in the attempt to manage the growth and development of organic life in the universe. As civilizations rise and grow more complex, the Reapers believe they need to be harvested or destroyed to maintain a certain balance. They are like the enforcers of a very strict galactic order. Their back story is full of mystery and a sense of inevitability for the civilizations that become their targets.
In science fiction, 'Reapers' can refer to various things. For example, in the 'Mass Effect' series, the Reapers are a race of highly advanced, sentient machines. They are colossal in size and have a cyclical pattern of wiping out all advanced organic life in the galaxy every so often to preserve a certain order, as they see it. Their technology is far beyond that of most species in the game universe, and they are a major threat that the protagonist has to deal with.
For novels that were similar to full-time ghost officers in the city, one could refer to the following:
The protagonist of a classic horror novel was a ghost officer named Hu Bugui. He traveled between the human world and the ghost world in search of lost civilizations and treasures.
2 " Youyou Baishu " by Japan: The protagonist of a classic fantasy novel is a person named White Snake. He is a ghost officer responsible for performing various strange missions.
[3] The author of << The Story of the Shepherd >> Wang Yu: The protagonist of a fantasy novel is a young man named Mu Yunsheng. He is a ghost officer who is responsible for carrying out various terrifying missions.
4 "Thriller Paradise" author Morpheus: The protagonist of a science fiction novel is a teenager named Li Siyuan. He is a ghost who uses his skills and mysterious power to solve various horror incidents.
5 " Full-time Reaper " Author Momo: The protagonist of a short story is a Reaper named Ye Fei. He performs various terrifying missions in the city. The story is compact and thrilling.