Draco Malfoy is a prime candidate. He has done a lot of bad things under the influence of his family and the Death Eaters. In this fanfic, he might be one of those who deeply feel regret for his actions, such as bullying Harry and his friends or being involved in some of Voldemort's schemes.
Severus Snape, although his actions were mostly in the service of a greater good, he may still have moments of regret. He had to be a double agent and in the process, he may have done things that he feels guilty about, like being cruel to Harry at times. In this fanfic, his sense of regret could be explored more deeply.
Another theme might be the nature of guilt itself. It could explore how guilt affects a person's magic or relationships within the Harry Potter universe. Maybe a character's guilt is so strong that it starts to manifest in strange ways, like causing their spells to go awry.
Hermione would surely be one. She's smart and would be great at helping with the academic side of building a school, like creating the curriculum.
Ron Weasley could also be a main character. He might be dragged into some adventure in the Prefects Bathroom by Hermione or Harry. For example, he could accidentally set off a magical trap in the bathroom and they all have to figure out how to get out of it without getting into trouble with the teachers.
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Since I don't know the specific 'billionaires regret novel', it's hard to say. But it could be the billionaire himself or herself as a main character, along with people from their past like old business partners or estranged family members.
Dumbledore would also be affected. As a mentor - like figure to Harry, he has certain expectations and plans for Harry. If Harry stops caring, Dumbledore's grand schemes against Voldemort might be derailed. He would have to find a new way to motivate Harry or adjust his strategies.
Hermione and Ron could be main characters too. They've been through so much with Harry, and their knowledge and skills would be very useful in new adventures. Also, there are other characters like Neville, who has grown so much, and he could play a major role in the new stories. There might even be new characters that are introduced and become central to the plot.
Maybe Harry accidentally cast a spell on Hermione during a training session that caused her some harm. So he feels guilty because he didn't mean to hurt his friend.
Well, Gandalf would probably be a main character too. He is a powerful wizard in Middle - earth and his knowledge and guidance would be crucial in a story where Harry is involved. He could be the one to help Harry understand the differences between his magic and the magic of Middle - earth. Also, Aragorn could be part of it. His leadership and warrior skills could be something that Harry looks up to or tries to learn from in this fanfic.
It could be Voldemort still. Since Harry is dabbling in dark magic by making a Horcrux, Voldemort might sense this and try to use it to his advantage. He could either try to steal Harry's Horcrux or manipulate Harry further into the dark side.