In some fanfiction, an outside force like a criminal gang could be responsible. If the gang has a grudge against the hospital or is looking for drugs stored there, and Meredith gets in their way during their raid, they could hurt her. Also, it could be a supernatural entity if the fanfiction has such elements, like a vengeful spirit that targets Meredith for some reason.
It could be a new, inexperienced doctor. Maybe they make a mistake during a surgery that Meredith is involved in, accidentally causing her harm. For example, they might misread a vital sign or use the wrong instrument.
A jealous colleague might be responsible. If they see Meredith as a threat to their own career or a relationship they desire, they could set up a situation where she gets hurt. They might, for instance, tamper with some medical equipment that Meredith is about to use.
It could be a rival assassin. They might see Kassandra as a threat to their own reputation or contracts.
Another possibility is a jealous student at Hogwarts. There are always students who might be envious of Hermione's academic achievements. They could set up a prank or a trap that goes wrong and ends up hurting her. For example, a student who is tired of Hermione always getting the top grades in Potions might tamper with her cauldron during a class experiment, resulting in an explosion that hurts her.
As I mentioned before, a member of his own wolf pack. In a pack, there can be jealousy and power struggles. If another wolf wants more influence or has a grudge against Jacob, they might be the ones to cause him harm.
It could be an enemy that manages to sneak onto the Argo 2. Maybe a monster that they thought they had defeated before but actually just bided its time and then attacked Percy when he was least expecting it.
There's a possibility that it's an old enemy who has somehow regained power or found a new way to attack. For example, Frieza's family or the Red Ribbon Army if they had some kind of resurgence. They might target Vegeta and Bulma out of revenge or to gain access to Capsule Corp's technology which Bulma controls.
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.
In some cases, it could be herself. Maybe she pushed herself too hard in her work or training, and her body or mind couldn't take it anymore. This self - inflicted harm could be a key part of the fanfiction's plot, as she then has to deal with the consequences and learn from her over - zealousness.
It could be an enemy from the underworld. Maybe a powerful demon or a vengeful spirit that has a grudge against her. They might use their dark powers to harm her, like sending a bolt of dark energy her way.
It could be a stranger. Maybe a new kid in town who is a bit of a troublemaker doesn't like Eric for some reason and decides to pick a fight with him.