Since I don't know the novel well, it's hard to say for sure. But likely, the dragon and the flower (or a character related to the flower like a flower spirit) are main characters.
It's possible that the main characters are a dragon with unique powers and a flower that has a special significance in the story's world. There could be a human character as well who is somehow connected to both the dragon and the flower. This human might be a mediator between the two different entities, or perhaps has a special ability to communicate with them. And there could be an antagonist, perhaps another powerful creature that wants to use the flower for its own evil purposes and is in conflict with the dragon.
I don't know the 'flower and the dragon novel' well enough to say who the main characters are. It could be the flower itself if it has some sort of sentient or magical form, and the dragon, of course.
I'm sorry I don't know exactly who the main characters are in this novel. But it's likely that there is a protagonist who is somehow related to the dragon flower. Maybe it's a young adventurer or a mage.
One of the main themes is probably adventure. The characters likely embark on various quests which is common in such novels.
I'm not entirely sure without reading it fully, but I bet there's a detective - type character who is trying to solve the mystery of the killings. There must also be the victims, who are likely members of the community in the 'Flower Moon' setting.
I don't know the main characters in this novel as I haven't read it. Maybe there's the king himself, and perhaps someone who is interested in the forbidden flower, like a gardener or a curious noble.
I'm not sure exactly, but it could be the florist who owns the shop. They would likely be a central figure, dealing with all aspects of running the business and interacting with customers.
The main characters in 'flower girl visual novel' may include the flower girl, of course. She might be joined by her family members, such as her mother or sister who also work with flowers or support her in some way. Additionally, there could be a rival in the flower business, a mean - spirited person who tries to steal her customers or sabotage her. And then there could be a mentor figure, perhaps an older florist who teaches her the tricks of the trade and also gives her advice on life.
There are also some supporting characters like the main female character's family and friends. Her family provides a contrast to the wealthy world of the F4. And her friends play roles in her life, either supporting her or sometimes causing trouble in her relationships with the boys.
There might also be side characters from the original that get more focus in the fanfiction. For instance, a servant who has a unique perspective on the events happening around the main characters and plays a more important role in this new fictional version.
There could be a wise old gardener who knows the secrets of the forbidden flower. The king, of course, would be a key character. And perhaps a young maiden who has a special connection to the flower, either because she can sense its power or she has a prophecy related to it.