The commoner she wants to marry is also crucial. He represents the 'other' in the story, from a different social stratum than the villainess. Their interactions and relationship development are central to the story.
There may be other characters like the villainess's family members or the nobility who oppose her decision. These characters create conflict and obstacles for the villainess in her pursuit of marrying the commoner, and thus are also important to the overall story.
Another theme might be self - discovery. As she tries to marry a commoner, she may learn more about herself, her desires, and what truly makes her happy outside of the expectations placed on her as a villainess.
The villainess herself is often a popular character as readers are interested in her journey from being 'bad' to potentially 'good'. The male lead, who is usually involved in the villainess's romantic arc, is also popular.
Obviously, the villager and the villainess are the main characters. There might also be other supporting characters like the villagers who oppose the idea of saving the villainess, or some characters related to the villainess's past who try to stop the villager from his mission.
There could be antagonists too. Maybe there are powerful nobles or some self - righteous individuals who are determined to keep the villainess in her 'bad' position. These antagonists would create more obstacles for the villager in his or her mission to save the villainess. The villainess herself is also an important character. She may start off as a seemingly cold or mean person but as the story progresses, her true nature is revealed through the villager's efforts.
Since I haven't read it, I'm not certain. But surely Lady Rose is one of the main characters as the title implies.
Another possible main character could be a love interest. If the story has a romantic sub - plot, this character could be either a commoner who Lady Rose falls in love with during her journey to become a commoner, or a noble who tries to win her back to the noble world. Their relationship could add more depth and complexity to the story, as they deal with the differences in their social statuses and Lady Rose's determination to change her life.
Since I haven't read it, I can't say for sure. But it's likely that the villainess herself is a main character. And of course, the beast she tames would also be a key character.
The villainess herself is surely one of the most interesting. Her determination to break away from the expected villainous path and seek peace makes her stand out. Maybe there's also a love interest who is interesting because he has to deal with her new - found goal of peace instead of the usual villainess behavior. And perhaps a sidekick or a friend character who helps her in her journey to live in peace.
The protagonist who favors the villainess can also be quite interesting. Their reasons for siding with the villainess, whether it's because they see something special in her or they want to change the story's course, can add depth to the character.
I think people who enjoy romance and drama-themed mangas would be interested in reading 'Who Wants to Marry a Billionaire'. The title itself suggests a captivating plot with potential for lots of twists and turns.