A fanfiction might focus on Doris' relationship with the people in her village. It could show how her life was before D's arrival, her daily routines, and how she was seen by the villagers. For example, perhaps she was a bit of an outcast because of her family's past, and this could add more depth to her character.
The number of Vampire Hunter D novels varies depending on different editions and releases. Generally, there are around a dozen or so. However, this can change over time with new additions to the series.
There are numerous Vampire Hunter D novels. The series has been ongoing for some time and has added many volumes over the years. It's hard to give an exact count as new ones might be released, but it's a fairly extensive series.
Friendship and loneliness are themes as well. D is often a solitary figure in his hunting, but sometimes he forms unexpected friendships or alliances. Some fanfictions explore how these relationships affect D and his mission. There is also the theme of the unknown and the supernatural world that D is constantly exploring, which adds an element of mystery to the stories.