In some comic versions, the Joker's scars origin is a mystery or has multiple conflicting explanations. But in one common story, he claims they were from a traumatic event in his past.
The origin of the Joker's scars varies depending on the story. Sometimes it's due to a chemical accident; other times, it's part of a more complex and darker backstory.
In 'The Dark Knight', the Joker gives two different versions of how he got his scars. One version involves his abusive father. Another is related to his wife. However, it's hard to tell which one is true as the Joker is known for his chaos and deception.
Well, it could be that in the fanfiction, Percy gets into fights with Gabe over protecting his mother. Gabe, being the brute he is, might use a weapon or his fists, and Percy ends up with scars as a result. For example, he could try to stop Gabe from hitting his mother, and Gabe turns on him, causing injury that leaves scars.
He was defeated by a combination of the superheroes' teamwork and strategic planning. Different heroes used their unique powers and skills to outsmart and overpower him.