Another possible antagonist could be a rival in love. There might be someone else who also desires the witch's beloved and would go to great lengths to break up their relationship. This character could create a lot of drama and tension in the story by plotting against the witch and trying to win over her beloved.
The main theme might be undying love. Since the witch is said to love her beloved until her dying day, it could explore the depth and permanence of love, perhaps in the face of various obstacles and challenges.
The witch is a central figure. Her beloved could be someone she has known for a long time, like a childhood friend. This would add a sense of history and depth to their relationship. There might also be antagonists in the story, such as other witches who envy her love or humans who are afraid of their relationship. These characters would create conflict and drive the story forward.
Since I haven't read it, I can't say for sure. But it's likely that the witch and her beloved are the main characters.
It could be about the power of love. The witch's love for her beloved might be so strong that it endures till the end. It may explore how her love overcomes various obstacles and difficulties.
Perhaps it's about a witch who has made a vow or has a natural inclination to love someone deeply and unconditionally until her last day. It could also be about the sacrifices she makes for this love, or how the society in the story's setting reacts to this relationship between a witch and her beloved.
One possible reason could be a vampire attack gone wrong. Maybe a rogue vampire targeted Bella and inflicted a near - fatal wound. Another reason might be related to some sort of dark magic or a curse that was unknowingly placed on her. It could also be due to a self - sacrifice situation where she put herself in harm's way to save someone she loves, like Edward or one of her friends.
Jealous characters could be antagonists. Maybe there are other girls who are envious of the beloved and pampered girl and try to bring her down. Another possibility is that her over - protective family members, in a sense, could act as antagonists if their pampering restricts her freedom too much.
One possible reason could be a curse that was placed on her secretly. Maybe an enemy from her past, someone she thought was defeated, managed to put a slow - acting curse on her. Another reason might be a side - effect of some powerful magic she used to save someone or solve a difficult problem. For example, if she used time - traveling magic in a wrong way, it could have negative consequences on her health.
It could be about mortality. Since it has 'last day' in the title, perhaps it explores the idea of facing one's end, like coming to terms with death, regrets, or unfulfilled dreams.
Another possible antagonist could be a character from Naruto's own world who has also been brought to this new world and has been corrupted by the power of the Witch King. This character might envy Naruto's new - found power as the Witch King and wants to take it for himself. So, he teams up with the dark forces in this new world to oppose Naruto, creating a lot of challenges for Naruto to overcome.