There was a menacing sea monster in the deep ocean. It spotted a mermaid swimming alone. The monster forced the mermaid to stay with it. The mermaid was heartbroken as she was separated from her family. But she was also brave. She started singing a very special song that could put any creature to sleep. The sea monster fell asleep, and the mermaid swam as fast as she could back to her underwater kingdom.
One possible reason could be some sort of external threat. Maybe there's a dark force that wants to use his mate against him, so to protect her, he's forced to reject her. For example, if there are dark wizards who plan to capture his mate and use her as a bait to get to Harry, he might think rejecting her would keep her out of danger.
One possible ending is that the victim manages to escape. Just like in the story of the captured fairy by the goblin. The fairy used her magic to blind the goblin momentarily and flew away.
It could be the Death Eaters. Even if they are on the 'bad side', they might see an opportunity to disrupt Harry's life and weaken him. By forcing him to reject his mate, they can create emotional turmoil for Harry, which would make him less focused on fighting against them.
It could be Hermione. In some alternate interpretations, there could be a situation where Hermione, while trying out a new and powerful magic, accidentally hurts Harry. Their friendship is put to the test as Harry has to deal with the pain and Hermione has to come to terms with what she's done.
Maybe it's Ron. Suppose Ron gets jealous of Harry's fame and power and, in a moment of weakness and envy, decides to abandon Harry to die. This would create a lot of drama as Ron and Harry have been friends for a long time, and such a betrayal would be a huge shock to the story's readers.
Well, it could be a completely new character created specifically for this fanfiction. Maybe an elf or a human from Middle - Earth who has some sort of special connection with Harry.
Some characters might be initially shocked. They could be confused about what it means and what implications it has for their future. For example, a character who values their independence might feel a bit trapped at first.
Mate choosing series novels often reflect real - life mate selection by showing the importance of values. For example, in many novels, characters are attracted to those who share similar moral and ethical beliefs. This is also true in real life, where people tend to look for partners with compatible values.
It could be a new character entirely. Maybe a hybrid of some sort, half - vampire and half - werewolf, which would be a very unique and interesting choice for her true mate. This would bring a whole new set of dynamics to the story as Bella has only been involved with either vampires or werewolves before.