He might be a mix of flattered and bewildered. Flattered because people care enough to write about him, but bewildered by some of the wild ideas in the fanfiction.
Percy would likely have a range of reactions. At first, he might be excited to see that his story has inspired others to write. But as he reads more, if he comes across some fanfiction that goes against his values or misrepresents his friends, he could get angry. However, if he finds some that are really positive and creative, he would probably enjoy them and might even be inspired to think about his own story in new ways.
He'd probably be shocked. I mean, Dudley has always seen Harry as the odd one out in the family, and suddenly finding out he's famous in a whole different world of fanfiction would be quite a surprise.
Annabeth would be completely devastated. She would probably go into a state of shock at first, then be filled with anger and determination to find a way to bring him back, if possible. She might even go on a solo quest to try and reverse his sacrifice.
He may feel a bit weird. Fanfiction can be so different from his real self. He might not like some of the wild ideas people have about him.
He might be intrigued. The fanfiction might show him a new side of the Greek mythological world that he's part of.
Snape might be initially shocked but then try to hide his concern. He's a complex character. He could act indifferent in public but secretly start to gather information about who is behind the abuse. He might also start to keep a closer eye on Harry without the boy realizing it.
He might be really surprised. Fanfiction can present him in all kinds of new situations that he never thought of. He could be amused by some of the creative interpretations of his character and his adventures.
The characters' reactions would vary widely. Annabeth would likely be critical of any fanfiction that got the details of her character or her knowledge wrong. She's very intelligent and detail - oriented, so inaccuracies would bother her. On the other hand, if a fanfiction captured her essence well, she might secretly enjoy it. Leo would probably be excited to see all the creative ideas people had in fanfiction, especially if there were cool new inventions or adventures in them.
Well, in many such fanfictions, Harry might be shocked at first. He probably feels a sense of pity for Draco too. After all, being under the Imperius Curse means Draco has no control over his actions.
He'd probably be annoyed. Snape is a serious wizard and he might think fanfiction is a silly Muggle - influenced concept.
He might be curious. Harry is always eager to learn new things. He'd probably read it with great interest to figure out what this new world of RWBY is all about.