Not having read the 'the succubus short story', I can only make assumptions. The succubus is surely a central figure. There may be a group of adventurers in the story. One of them might be the main target of the succubus. And there could be a local guide or a seer who warns the group about the presence of the succubus and its danger.
The main character is often the succubus herself. There may also be other demons, like powerful lords of the underworld who could be either her allies or enemies. Sometimes, there are also mortal characters who get involved in the story, either by accident or because they are lured by the succubus's charm.
Obviously, the succubus would be one of the main characters. She could be joined by other demons, like her friends or enemies in the demon world. And then there would probably be some human characters. For example, a priest who tries to exorcise her or a young man who is attracted to her despite knowing what she is. These characters would drive the plot forward through their interactions.
The main character is surely the succubus. However, there might be other important characters such as a human she has a special connection with. This human could be either a target of her seduction or someone who she forms an unexpected bond with. There may also be an antagonist, perhaps a powerful demon who wants to control her or a hunter who is after her.
Obviously, Bella is the main character as she is the succubus. There might also be a love interest, perhaps a human or another supernatural being. For example, in some fanfics, there could be a vampire character who becomes intrigued by Bella's succubus nature.
Obviously, Harry would be the main character as the title indicates his transformation into a succubus. His friends like Hermione and Ron are likely to be important secondary characters. They would play a role in either helping Harry come to terms with his new nature or being initially suspicious of the changes in him.
I haven't read this specific 'the succubus short story', so I can't summarize it directly. But generally, a succubus in stories is often a female demon. Maybe the story is about her interactions with humans, her powers, or her role in a particular fictional world.
Based on the title, the main characters are likely to be the succubus and the witch.
Obviously, from the title, a succubus and a witch are likely to be main characters.
Well, in addition to Naruto and the succubus, there could be other characters from the Naruto universe. For instance, his friends like Sasuke or Sakura might make an appearance. They could either help Naruto in his fight against the succubus or be used by the succubus in some way to try and manipulate Naruto. Also, Kakashi Sensei could potentially show up to offer some sage advice to Naruto on how to deal with this new and strange threat.
Well, there could be several themes. For instance, the theme of acceptance. The succubus may have to accept the Christmas traditions and values in the story, which are quite different from her own nature. Additionally, the theme of unity might be explored. In the Litrpg world of the story, different characters, including the succubus, might come together during Christmas, despite their differences, to achieve a common goal. This could be related to a game - based objective like saving the Christmas - themed world from some sort of threat.