One of the most interesting characters can be the original heroine of the story. In 'reincarnated as a villainess fanfiction', she might be portrayed in a different light. Maybe she's not as perfect as she seemed in the original. The villainess's family members are also interesting. For example, a brother who is overprotective in the original story might become an ally in the fanfiction. And let's not forget the mysterious wizard or mage characters who might have a role in the reincarnation process or help the new - turned - good villainess along the way.
The protagonist, who is the reincarnated villainess, is of course a main character. She's the one driving the story as she tries to change her fate. Then there's usually a male lead, who is often part of the original story that she's now trying to rewrite. There's also the original heroine, with whom the protagonist has a complex relationship as she's now in the role of the villainess against her. These three are typically the main characters that shape the story.
One important character could be the villainess' former rival or enemy from her past life. Their interactions in this new life could be full of tension and drama. Also, there may be a mentor - like figure, perhaps an older witch or a wise noble, who helps the villainess understand her past and change for the better. And of course, her family members, whether they are supportive or part of her past problems, are important characters too.
One main theme could be redemption. Since the protagonist is a reincarnated villainess, she might be trying to atone for her past wrongs in her previous life. Another theme could be power struggle, as she might have been involved in complex power dynamics in her dark history.
One common plot device is the protagonist using her knowledge from her previous life to avoid the original villainess's bad end. For example, she might change her attitude towards the male lead or other characters. Another is the discovery of hidden powers or abilities that the original villainess didn't know about, which helps her gain an edge in the new world. Also, the creation of new relationships that were not present in the original story, like befriending characters who were once enemies in the original plot.
First, you need to come up with a unique and interesting character design for the villainess. Make her have complex motivations and personality traits. Also, think of a captivating plot that keeps readers engaged.
Well, 'A Villainess for the Tyrant' manga is appealing because it offers a fresh take on the typical romance tropes. The world-building is also quite impressive and draws you in.