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.
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.
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.
Peter is the central figure in this fanfiction. He's the one who has to deal with the symbiote. But also, his friends like Ned Leeds and MJ might play important roles. They could be the ones who notice the changes in Peter due to the symbiote and try to help him or at least figure out what's going on. Maybe Flash Thompson could also be involved, either as an antagonist or someone who gets caught up in the chaos that the symbiote - influenced Peter might cause.
I don't know as I haven't read the book. But I assume there must be a female character who holds the secret. There might also be other characters like her friends, family members, or a love interest who are affected by or somehow involved in her secret.
Since the title is 'the secrets she keeps novel', the main character is presumably a woman with secrets. There could be other supporting characters such as her confidant, someone who tries to discover her secrets, or those who are unknowingly affected by her secrets. For example, a neighbor who notices her strange behavior due to the secrets she is hiding.