It could be some jealous villagers who still hold grudges against Naruto for having the Nine - Tails inside him.
In some cases, it could be an old enemy of Naruto's father, Minato. They might take out their anger on Naruto because of his relation to Minato. Or perhaps it's a group of bullies within the ninja academy who are jealous of Naruto's growing strength and potential, and they take it too far in their attempts to put him down.
Well, in such a fanfiction, Kakashi might be really angry when he discovers Naruto has been abused. He could use his skills to protect Naruto and find out who the abuser is.
In some cases, it could be a group of jealous fans. They might abuse Zane verbally, by booing him during duels or spreading mean - spirited comments on dueling forums. Their collective hatred could be a form of abuse that really affects Zane's confidence and performance.
In this fanfiction, perhaps Naruto confronts Kushina about the secrets she kept from him. For example, if there were things she knew about his powers or his destiny that she didn't share before her death. He could be feeling betrayed and wants answers.
There are many possibilities. It could be a character from Slytherin who has a long - standing grudge against Harry. They might be using abuse as a means to undermine Harry's confidence and disrupt his relationship in the romance aspect of the fanfiction. Another option could be a dark wizard in hiding who has a personal vendetta against Harry and sees abuse as a way to torment him without attracting too much attention from the wizarding community.
Well, Kakashi is a complex character. When he discovers Naruto is abused in the fanfiction, he may first feel a deep sense of guilt for not noticing earlier. He'd then use his skills and influence in the village. He could start by reporting it to the proper authorities in Konoha, like the Hokage. And at the same time, he'd be there for Naruto, providing emotional support, talking to him, and helping him through the trauma. Kakashi has his own past traumas, so he'd understand what Naruto is going through on some level.
It could be a new group of rogue ninjas who oppose the ideals of the Hidden Leaf Village. They might target Kakashi's daughter because of her connection to Kakashi and Naruto, seeing her as a symbol of the village's new generation of strength.
Another option could be Itachi. Although Itachi is from a different village and has his own complex situation, in a fanfiction scenario, he could secretly train Naruto. Itachi has extremely powerful jutsus and a deep understanding of the Sharingan. Training under him would give Naruto a very different perspective on power and combat, not to mention some insights into the Uchiha clan.
It could be due to a misunderstanding. Kakashi might think that Naruto is getting involved with something dangerous or has some potential that he can't handle. So, in his warped way of thinking, he abandons Naruto to protect him from himself or from some unknown danger.
They could also face legal consequences in the wizarding world. There are laws in place to protect the rights of wizards, and abuse is likely a violation of those laws. The Ministry of Magic might get involved, and the abuser could be sent to Azkaban or face other forms of punishment depending on the severity of the abuse.
The abuser could face immediate death. Gods like Hades might be quick to drag the abuser down to the Underworld for punishment.