In many fanfictions, it could be Ginny. Since they have a relationship in the original series, it would be a shock if she were to cheat on Harry. It would create a lot of drama and give the writer a chance to explore Harry's vulnerability.
He might cry a lot. Deku is very emotional, and being cheated on would be a very painful experience for him. He could also get angry, which is rare for him, but this situation would be so unjust that he might not be able to hold it in. He'd probably start to question his self - worth too, thinking that there was something wrong with him that made his partner cheat.
There's also a chance that Neville Longbottom could be involved in a cheating situation. He might have grown into a very different person in the fanfiction, and due to some complex emotional and social factors at Hogwarts, he cheats on his significant other. This would be quite a shock as Neville is usually seen as a very loyal character.
Well, it could be a rival of Ciel's in business or in a social setting. Maybe someone who is jealous of his status or wealth and sees shooting him as a way to get ahead.
Maybe a secret admirer who is too shy to approach her normally. They think that by kidnapping her, they can have some alone time with her and confess their feelings. However, this is a very extreme and wrong way to go about it. Another possibility is a group of thieves who think Haruhi comes from a wealthy family and they can get a big ransom for her.
In some fanfictions, it could be an unknown alien species. They might have detected Bumblebee's unique energy signature and decided to take him to study his technology or abilities. This could lead to all sorts of interesting adventures as Bumblebee tries to communicate with them and find a way to escape.
It could be an old enemy of Ezra. Maybe someone from his past who has a grudge against him for some reason, like a business deal gone wrong or a personal feud.
Ron might support Harry at first. Since he's Harry's best friend, he would be on Harry's side without really knowing the full story. He'd probably offer to help Harry with some of his revenge plans, like distracting Ginny while Harry sets up a magical prank.
Ron would also be there. He might start out a bit reluctant, but then get into it. He could be Harry's partner in physical challenges, like having races on broomsticks or wrestling magical creatures in a controlled environment for fitness.
His friends from Hogwarts like Ron and Hermione. They are always by his side and would probably be more than happy to help Harry get revenge on the Dursleys. They might use their magic skills together to create some really interesting revenge scenarios.
Ino could be a very complex character in this fanfiction. It's possible that she cheats on Naruto because she has a fear of commitment. Naruto is a very passionate and all - in kind of person, and that might scare Ino. So, to avoid getting too deep into the relationship, she cheats. Or perhaps there's a power struggle at play. She might think that being with a different person would give her more influence or status in the ninja world. And there could be a hidden agenda that she's following, something that she can't achieve with Naruto but believes she can with someone else.