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.