It could also be Harley herself in a way. Her own recklessness might lead to her getting hurt. For instance, she might jump into a dangerous situation without proper planning.
A new villain in the story could be responsible. This villain could be out to make a name for themselves by taking down Harley.
Sometimes, it could be due to her own actions. Lydia might be overconfident and take risks that she shouldn't. For instance, she could try to perform a very difficult and dangerous magic spell without proper training, and it backfires, causing her to get hurt.
It could be a villain in the fanfiction. Maybe a new, powerful vampire who has a vendetta against Bella or her friends.
Another character who might be responsible is a jealous rival. This person could be envious of Elena's popularity or her relationship with someone important. So, they set a trap that causes her to get hurt.
In the fanfiction, it might be someone from his own team. Maybe there's a new member who is insecure and feels that Kuroko is getting too much attention. So, in a fit of jealousy, this person does something that causes Kuroko to get hurt, like tampering with his equipment.
Perhaps an unknown ice sorcerer. Since the Frozen world has magic, a new character who is jealous of Elsa's power could target Anna to get to Elsa. They might use their ice powers to create dangerous situations for Anna, like making the ice floor slippery and causing her to fall and get hurt.
It could be Vilgax. He's one of Ben's most powerful enemies. Vilgax is always looking for ways to defeat Ben, and he might come up with a new and devious plan to hurt him. Maybe he teams up with other villains to ambush Ben when he's least expecting it.
It could be an enemy. For example, a rival in a competition or a villain who has a grudge against Leo.
Maybe it was an accident caused by one of his own team members. For example, if Chase was working on a new invention and there was an unforeseen error that affected Adam when he was nearby. It wasn't intentional, but it still led to Adam getting hurt.
It could be a rival from another house. For example, a jealous Slytherin who wants to take Ginny out of the Quidditch game. They might use some underhanded tactics to make her broom malfunction and cause her to get hurt.
Maybe it's a jealous acquaintance. If Kagami is really good at something, like she's a star in a particular activity, someone who is jealous of her success might set up a situation where she gets hurt. For instance, they could tamper with her equipment in a way that causes her to have an accident.