Obviously, Harry Potter would be one of the main characters. He is the one calling in the life debts. There might also be the people who owe him life debts, like some of his friends who he has saved in the past, or even some unexpected characters from the wizarding world.
Harry Potter is sure to be a main character as the title implies he is the one involved in calling life debts. Then, there could be characters like Ron and Hermione. They have been through so many adventures with Harry and he might have saved their lives at some point, making them potential characters who are involved in the life debt situation. Also, there could be some villains or antagonists who are affected by Harry's calling in of life debts, for example, a Death Eater who Harry spares and who then owes him a life debt.
There could also be characters like Dumbledore, even if in spirit or through memories. His wisdom might have influenced Harry's understanding of life debts, and his past actions could be related to debts that Harry is now calling in. And then there are the characters who owe Harry their lives. This could include Neville, who Harry has saved on multiple occasions, and their relationship could be further explored in the context of the life debt.
Obviously, Harry Potter would be the main character as the title implies. But his friends like Ron Weasley and Hermione Granger are also very likely to be important characters. Since they've been through so much with Harry, they might be among those who are called upon to pay their debts, or they could help Harry in the process of calling in debts from others.
Obviously, Harry Potter himself is the central character. Others could include Ron and Hermione, who have been by his side through many life - threatening situations and might also be in debt to him in some way, for example, if Harry saved them from a particular dark spell during a battle. Also, Neville Longbottom could be involved as Harry saved him from Voldemort during the Battle of Hogwarts.
Well, obviously Harry Potter himself would be the central character. He has always been a fighter for what's right in the original series, so it makes sense for him to be the main character in a fanfiction about justice.
One possible plotline could be Harry using life debts to form powerful alliances. For example, he might call in a life debt from a powerful wizard who then helps him in a battle against dark forces. Another could be that Harry uses a life debt to gain access to some long - hidden knowledge or a secret place.
In addition to Luffy and Garp, other characters could also play important roles. For example, some of Luffy's crewmates like Zoro or Nami might be present when Luffy calls Garp. They could add their own perspectives and reactions. Also, there could be some marines around Garp when he receives the call, and their reactions to Garp's decision to go to Luffy could also be part of the story.
Hermione Granger might also be a common character in such fanfictions. Given her intelligence and the times she has been in dangerous situations, there could be scenarios where someone owes her a life debt. For instance, a character she saved from a dark wizard might then be bound by a life debt to her, and their relationship could be explored in the fanfiction.
Obviously, Harry Potter himself is a central character. There could also be members of the families involved in the blood feud. For example, if it's a feud between two pure - blood families like the Malfoys and the Weasleys, we would expect to see Draco Malfoy and Ron Weasley in the story. And of course, Hermione would likely be by Harry's side, using her intelligence to help him figure things out.
Perhaps Harry Potter uses the life debts to reform the wizarding world. He calls in these debts to get support for new laws and regulations that would benefit the marginalized groups in the wizarding community. He could ask those indebted to him to vote for him in the Wizengamot or help him spread awareness about his new ideas.
Another plot could involve Harry calling in life debts from multiple characters at once to mount a large - scale defense against a new threat. These characters, bound by their life debts, would have to put aside their differences and work together. This would showcase Harry's strategic thinking and also explore the complex relationships between different characters in the fanfiction world.