Love, obviously. It's in the title. The love between a human and a ghost.
The theme of mystery is likely present too. Ghosts are often mysterious beings. In this novel, there could be things like the origin of the ghost, why they are still lingering around, and what their true intentions are in the relationship. All these add to the overall mysterious atmosphere of the story.
Another theme could be the supernatural. Since it's about a ghost, there are elements like the otherworldly existence, the different rules that ghosts might follow compared to the living. For example, how they interact with the physical world or how they experience emotions in a different way. It also might touch on themes of acceptance, as the human in the story has to accept the ghost's existence and nature in order to have a relationship with it.
Another possible theme is the exploration of the unknown. The boy next door might be somewhat of an enigma at first, and the main character's journey to understand him represents the exploration of the unknown. This theme could also be related to the idea of stepping out of one's comfort zone and experiencing new things.
Love might be one of the main themes. Often in novels with a title like this, there's a relationship aspect that involves love in some form, whether it's romantic love or platonic love between characters.
Friendship could also be an important theme. The characters might develop strong friendships within the neighborhood, which add depth to the story. For example, the boys in the story could be friends and support each other through various situations.
Maybe a young woman who lives in a house and the male ghost next door. The woman could be curious about the strange happenings in the neighboring house which are caused by the ghost, and that could be the start of their relationship.
It could be both. It might be a story where the presence of a ghost next door initially seems spooky (horror element), but then a love story develops between the main character and the ghost, which is the romantic part.
Another theme could be self - discovery. As the story progresses with the presence of this special boy next door, the main character might start to learn more about themselves, their desires and what they truly want in life. This could be related to how they interact with the boy and the situations that unfold.
Growth and self - discovery also play a role. The experiences with the boy next door can push the main character to learn more about themselves, their likes and dislikes, and how to handle relationships. For example, through misunderstandings or shared adventures with the boy next door, the main character might grow as a person.
One main theme is kindness. The 'angel next door' shows great kindness to the main character. Another theme could be growth, as the main character may grow and change due to the angel's influence.
Strange noises at night might make it a ghost story.
One possible theme could be friendship. Since it's about a girl next door, she might form deep friendships with the people around her.