Well, sometimes the plot twist is that the girl was actually faking the abuse to get attention, but then she really does end up in a dangerous situation. The boy, in his attempt to save her, discovers her true motives. Another possibility is that the abuser is part of a larger, more sinister group, and by saving the girl, the boy gets drawn into a much bigger conflict. This could lead to him having to save not just the girl, but also many others who are in danger from this group.
For the boy, his backstory can be developed to show why he is so brave and willing to save the girl. Maybe he had a similar experience in his past or was raised to be a protector. The girl can be developed by showing her strength in the face of abuse. She could start off as a victim but gradually become more assertive. For example, she might learn self - defense skills from the boy after being saved.
One possible scenario could be that in this fanfiction, Harry discovers that Hermione is being bullied by some dark - minded students at Hogwarts. He steps in immediately, using his skills and the respect he has among his peers to put an end to it. Maybe he threatens the bullies with exposure to the professors if they don't stop.
The boy could be a regular guy who just happens to be in the right place at the right time. He sees the girl being kidnapped and without thinking twice, he runs towards the kidnappers. He might throw something at them to distract them and then quickly grab the girl's hand and run. This simple act of bravery can be the start of a great adventure as they try to escape the kidnappers who are now chasing them.
He could use his position of authority. Snape might confront the abuser directly. Since he's a teacher, he has the power to intimidate and put a stop to any immediate harm. He could also give Hermione a safe place to stay, like his own quarters (if it's within the rules of the fanfiction world).
One possible plot could be that the boy, let's call him Tom, notices the girl, say Lily, being forced into a van by the kidnappers. Tom immediately springs into action. He chases after the van on his bike, using his knowledge of the local alleys to cut them off. He bravely confronts the kidnappers, using his wits and a bit of self - taught martial arts to overpower them and save Lily. It could be a story full of suspense and action.
They might face magical justice. In some fanfictions, the Ministry of Magic could get involved and strip them of their magic, or at least put them under strict surveillance.
Well, it could start with Hermione showing signs of distress. Harry notices and starts to investigate. He finds out that there's a group of Slytherins who are using dark magic to mentally abuse Hermione, making her doubt her own abilities. Harry then challenges them to a wizard's duel and wins, forcing them to stop their abuse.
There is also 'Boy's Heroic Save of the Lost Girl'. In this fanfiction, the boy uses his wits and courage to rescue the girl who is trapped in a strange place. It has an exciting plot with many twists and turns.