The main characters are obviously Alex and Kaiden. They are the focus of the fanfic and the story usually revolves around their relationship, whether it's a friendship, a partnership in some adventure, or something else.
In this Japanese ghost story, there might be a theme of cultural heritage. It could show how certain traditions and beliefs in Japanese culture are related to the existence of ghosts. Another theme could be the exploration of the unknown. Ghosts represent the unknown in many cultures, and 'Kaiden' could be about the characters' journey to understand and deal with this unknown entity, whether it's through fear, acceptance, or trying to get rid of it.
It's likely a collection of ghost stories. Usually, such works involve various spooky tales about ghosts, spirits, and the supernatural. It might include stories of haunted places, encounters with malevolent spirits, and the like.
Sadly, I don't know the creator of 'hyaku monogatari kaiden 100 ghost stories'. It might be a work that has evolved over time, with different elements added by different people. It could have started as oral traditions in some rural areas of Japan, and then as the stories became more popular, they were gathered together and named as such, but the exact originator remains a mystery.
Kaiden, as a Japanese ghost story, would probably have a unique cultural backdrop. Japanese ghost stories often draw from their rich history, folklore, and religious beliefs. In this story, there might be references to Shinto or Buddhist concepts. Maybe it has a yurei, a traditional Japanese ghost, which is often depicted as having long black hair and wearing a white kimono, and the plot could revolve around how people interact with this supernatural being.
One great Alex and Kaiden fanfic could be 'The Adventures of Alex and Kaiden'. It might have them going on an exciting journey together, facing challenges and growing closer as friends. Another could be 'Alex and Kaiden: A New Beginning' which could explore their first meeting and how their relationship started.
The Jujutsu Kaisen manga is indeed ending. It's been a great run, but all good things must come to an end. The final chapters are bringing closure to the characters and story arcs.