Obviously, Charlie and Miles are the main characters. Charlie is the one who has the big discovery about Miles being her father. And Miles is the person who is revealed to be Charlie's dad, so he must have a big role in the story as well, with his own backstory and reasons for why this situation has come about.
Perhaps Charlie overhears a conversation that makes her suspect Miles is her father. In the fanfic, the story could then focus on how Charlie approaches Miles to confirm this. There could be a lot of internal struggle for both of them. Miles might be hesitant to admit it at first, fearing how it will change their relationship. Charlie, on the other hand, might be desperate for answers and a real connection with her father she never knew she had.
He might be shocked at first. In many fanfictions, Adrien would feel a mix of disbelief, anger, and sadness. He has always known his father as a strict but loving parent, and finding out he's Hawk Moth, a supervillain, would completely shatter his view of him.
That could be a really interesting situation. Maybe they are sharing a moment of bonding over exciting tales. The mom might be happy to see them so engaged.
They might be really surprised. Some characters could be flattered by the positive portrayals in the fanfiction. For example, Mirabel might be happy to see different interpretations of her story and how she's seen as a hero in some fics.