Harry Potter himself could be the main character. Given his history with Voldemort and his Death Eaters, it would be interesting to see him try to get closer to an enemy to gain an upper hand in the fight against the Dark Arts.
Perhaps the fanfiction could explore how keeping an enemy closer affects the character's relationships with their friends. They might have to keep secrets from their friends, which could lead to some tense and interesting situations. For example, if Harry Potter were to befriend Draco Malfoy secretly, Ron and Hermione would be very suspicious and it could create a rift in their trio.
In general, close friends may share your story under certain circumstances. If the story is something positive like your achievements or a funny anecdote, they might share it to show their pride in you or just to make others laugh. But if it's a very personal or sensitive story, a good close friend should keep it to themselves. For instance, stories about your family problems or your insecurities. It's all about mutual respect and understanding in a friendship. If you've made it clear that a story is private, a close friend should abide by that.
To add to your close friends' story, you could come up with a twist or a new perspective. Have a chat with them about what they think would work and then go from there. It's all about collaborating and sharing creative thoughts.
Ideally, no. But in reality, some might. If they do, it can be a betrayal of trust. For example, if you confide in a close friend about a personal struggle and they share it without asking, it can make you feel violated. A true friend should value your feelings and keep your story confidential until you say otherwise.