Since it's part of the 'Vampire Hunter D' series, volume 7 novel would likely follow D's ongoing battle against vampires. It could explore more of D's past or his unique abilities. There might be new challenges for D, like a powerful vampire lord that he has to defeat. The story could also introduce new elements to the world - building, such as different types of vampire powers or new regions that D travels to. And it might also show how D's actions affect the relationship between humans and vampires in that world.
The most prominent character is surely D. He is the central figure in the 'Vampire Hunter D' series. In volume 7, he is probably still the one driving the story forward with his quests and battles against vampires. There may also be some supporting characters like the villagers he might be protecting or other hunters he might team up with, but D is the main focus.
The Hunter x Hunter manga is unfortunately not complete. The author has faced various issues that have led to intermittent breaks and an unresolved storyline.