I'm not sure specifically as there could be many different 'darkside graphic novels'. Generally, it might be about dark themes such as mystery, horror, or the darker side of human nature. It could involve complex characters dealing with difficult situations in a dark and often gritty setting.
Since I don't know the specific 'darkside graphic novel', it's hard to say. But typically, there could be a brooding protagonist, perhaps someone with a troubled past. There might also be antagonists who represent the forces of the darkside more blatantly, like a powerful and malevolent villain.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.