Dio Brando is an important and iconic antagonist. His complex relationship with the Joestar family, his thirst for power, and his own vampiric abilities make him a central figure in the visual novel's story.
One unique feature is its distinct art style which is very recognizable and adds to the overall charm of the visual novel. It often has flamboyant character designs.
The main distinction of porn with a story genre from regular porn is the presence of a coherent narrative. Regular porn often jumps straight to the sexual action, while the story genre tries to add some kind of story framework around it. But in the end, both are adult-oriented and feature explicit sexual content.
Well, it's hard to say for sure. The concept of 'porn-like' is quite subjective. Some might consider certain novels with excessive sexual descriptions as such, but it depends on individual perspectives and cultural standards.
In many places, 18+ porn comics are considered illegal. This is because they can have a negative impact on society and violate moral and legal standards.