Well, while Hagrid and Harry are the central figures, there could be other characters making appearances too. For instance, some of the magical creatures that Hagrid is so fond of might play a role. The hippogriffs or Norbert (the baby dragon) could be involved in some of the events during the summer, even if they are not as central as Hagrid and Harry.
I think Hagrid and Harry are the main ones. Hagrid's role is to be the caretaker and mentor of sorts for Harry during this summer. Harry, on the other hand, is the one who will be experiencing new things, learning from Hagrid, and having his own character development. There may be other minor characters from the Hogwarts world that could pop up, but Hagrid and Harry are the stars of this fanfic.
Obviously, Hagrid and Harry are the main characters. Hagrid, the half - giant with a big heart, is taking care of Harry for the summer. Harry, the young wizard, is in for an interesting time under Hagrid's care.
Maybe Hagrid will teach Harry all kinds of magical creatures in that summer. He could introduce Harry to the hippogriffs, how to approach them safely and gain their trust.
Dumbledore often makes an appearance in such fanfiction. He might be the one who gives Harry and Hagrid their special tasks or provides guidance. Another popular character could be Hermione. She could be the one who helps Harry and Hagrid with research or comes up with clever solutions when they are in a bind. Ron might also be there, adding some humor to the story and being Harry's loyal friend as usual.
Obviously, Harry Potter is one of the main characters. He is the one who is trying to keep Norbert. And Norbert himself is also kind of a main character in a sense, as the whole story revolves around Harry keeping him. Hagrid might also be an important character as he was the one who brought Norbert to Hogwarts in the first place.
Since it's a Harry Potter fanfiction, it could involve the main characters from the original series like Harry, Hermione, and Ron. Maybe they are the ones dealing with the situation of the babies being 'for keeps'.
Obviously, Harry Potter is one of the main characters as the title implies he is the one who keeps the Deathly Hallows. Hermione Granger and Ron Weasley are likely to be important as well, as they are Harry's closest friends and would probably be involved in whatever adventures or challenges come with Harry having the Hallows.
Obviously, Harry is one of the main characters. He's the one who has the Half - Blood Prince's book. Hermione might also be a main character as she often helps Harry understand the complex knowledge in the book. Ron could be there too, providing his unique perspective and support.
One possible reason could be that Sirius, being Harry's godfather, feels it's his right and duty to protect Harry. He might not trust Hagrid fully to keep Harry safe, especially considering the dangerous situation Harry is in. Sirius has a strong connection to Harry and wants to be directly involved in his upbringing and protection.
Yes, there may be some out there in the vast realm of fan - created content. However, this kind of fanfic often goes against the original intention of the 'Harry Potter' series which focuses more on friendship, mentorship and the battle between good and evil. J.K. Rowling has not intended for such a relationship in her original work.
I don't know as I haven't read the 'playing for keeps novel'. But usually, main characters are those who drive the story forward, like a protagonist who has a goal and an antagonist who tries to stop the protagonist from achieving that goal.
The consequences would be far - reaching. Firstly, Harry would be completely ostracized from the wizarding community. His friends would be horrified and would likely turn against him. He would probably be sent to Azkaban for such a crime.