The parents, Mom and Dad, would probably play a significant role as they are the ones who could potentially bust Candace. Also, there could be some minor characters from the original show like Perry the Platypus. He might be involved in some way, perhaps he accidentally reveals Candace's plans which lead to her getting busted.
Candace is obviously a very popular character as the whole concept often revolves around her. She's the one with the mission of busting Phineas and Ferb. Phineas and Ferb themselves are also really popular. Their crazy inventions and carefree attitudes are what drive Candace's need to bust them in the first place.
The fanfiction might also explore a situation where Candace gets busted by her brothers first. They could turn the tables on her by proving that she's been so obsessed with busting them that she's actually been the one causing some problems herself. This would create a lot of drama and character development for Candace.
A really cool plot might involve Candace teaming up with an unexpected character to bust Phineas and Ferb. For example, she could team up with Doofenshmirtz. They both have their own reasons for wanting to stop Phineas and Ferb's projects. But as they work together, they start to see things from a different perspective and might even end up becoming friends with Phineas and Ferb instead.
Most likely, Harry Potter, Hermione Granger, and Ron Weasley would be main characters. Since it's a fanfiction related to the Harry Potter series, they are the central figures in the original story and would probably play important roles in any fan - created plot where they are 'busted'.
Obviously, Italy itself would be the main character. There could also be other European countries like France and Germany. They might be the ones causing Italy's anger, or they could be Italy's allies trying to calm it down.
There could be her family members. Her mother might be the one constantly checking on her temperature and making sure she is comfortable.
Peter Parker is the main character for sure. In addition to Aunt May and MJ, there could be other characters from the Spider - Man universe. For example, Doctor Strange could make an appearance if the injury has some sort of magical or other - worldly aspect to it. Or perhaps Iron Man's influence could be felt if the story is set in a universe where they have interacted. And let's not forget the Daily Bugle crew, like J. Jonah Jameson, who might be looking for a story about Spider - Man's injury.
The main character is Steven. Alongside him, his father Greg could be a crucial character. Greg has always been there for Steven in different ways, and he might be the one who can reach Steven on an emotional level during his corrupted state. Lion, Steven's magical pet, could also be a main character. Lion has the ability to travel through dimensions and might be able to find a solution to the corruption in some far - off place.
The parents, Linda and Lawrence, would probably be important characters. Since Candace has snapped, their reactions to her new behavior would be a key part of the story. There could also be some new characters introduced, like a mysterious mentor who helps Candace come up with her plans or a bully who takes advantage of her new - found 'edge'.
There could be her parents. They might play a role in encouraging her to get braces or helping her take care of them. For example, they could remind her to brush her teeth properly with braces.