The ghosts of Hogwarts could also play a role. Nick, for example, might be called upon by Harry as Harry once helped him with something in the wizarding school. And some of the former Order of the Phoenix members who Harry saved during the fight against Voldemort could be part of the story, like Remus Lupin if Harry had done something extraordinary to save his life at some point.
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.
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.
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.
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 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.
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.
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.
In this fanfiction, perhaps Harry Potter has reached a point where he realizes he's done a lot for others in the wizarding community. So, he starts calling in all the debts. It could involve his friends like Ron and Hermione, who he might call upon for special assistance. It might also include other characters he's helped over the years. This could lead to all sorts of interesting plot developments, such as new alliances being formed or old secrets being revealed as people try to fulfill their debts to Harry.
Maybe it's a new character, a foreign wizard who got into trouble in the wizarding world of Britain. Harry saves this wizard's life, and when he calls in the life debt, the story could take a romantic turn as they explore each other's cultures. In the lemon part, it could involve some passionate encounters as they get to know each other better.
Even some of the professors could be involved. Professor Dumbledore, with his vast knowledge and wisdom, might have some connection to the magical trunk. He could be the one guiding the students in their exploration of it, or perhaps he has left some clues for them to discover. Professor McGonagall might also get involved if she senses that the students are in over their heads with the trunk's secrets and she needs to step in to protect them.
Neville could be part of it too. Maybe he sees Harry's training and gets inspired to improve his own magical abilities. He could learn from Harry's experiences under Sirius's training. And the Weasley twins might show up to add some mischief and perhaps even create some magical gadgets to aid in Harry's training.