Well, it could completely shift the power dynamic. Instead of being enemies, they become a sort of family unit. This might lead to Harry being taught dark magic rather than the traditional magic at Hogwarts. It could also mean that the Order of the Phoenix would see Harry as a threat rather than their hope.
In this fan fiction scenario where Voldemort adopts Harry, the story would be a radical departure from the original. Harry's identity would be in flux. He would be living in a world where the values of the Dark Lord are imposed on him. His relationships with other characters like Hermione and Ron would be non - existent or at least very different. Voldemort might groom Harry to be his heir, which would change the whole purpose of the fight against Voldemort in the original story.
Harry might become more conflicted. He has Voldemort as a sort of father figure, but he also has his own sense of right and wrong. So he could be constantly struggling between the dark path Voldemort wants him to take and his own moral compass.
Well, it completely changes the nature of the magic - based world. Usually, the battle between Harry and Voldemort is about magic spells and wizardry. With a gun, it brings in a Muggle weapon, which might make the wizarding world seem less special. It could also change the way other characters view Harry. Some might see him as a radical for using such an un - wizard - like method.
Well, in such a fanfiction, Harry being Voldemort's snake might initially make their relationship more of a master - servant one. But as the story progresses, Harry could start to resist Voldemort's control, leading to a more adversarial relationship.
Often in fanfiction, their relationship can change drastically. They could end up as allies, which would require both of them to change. Harry would have to see past Voldemort's evil deeds, and Voldemort would have to let go of his hatred for Harry. This exploration of character change makes fanfiction so interesting.
His reaction would likely be very different. At first, he might be angry that he didn't know this before. But then, he could start to see opportunities. He may think that if he can somehow control Harry as a Horcrux, he can gain more power. So, he might change his strategy from simply trying to murder Harry to trying to subdue him in a different way.
Harry's character could also become more isolated. His new relationship with Voldemort is not something that the wizarding world would easily accept. So he might find himself alone, with only a few people, if any, who are willing to support him. This isolation could make him more introspective, and he might start to rely more on his own judgment rather than the opinions of others.
Harry often becomes more conflicted. He has always seen Voldemort as the ultimate evil, but now being mates, he has to re - evaluate his views. He might start to see the humanity in Voldemort, which makes him question his own beliefs and actions in the fight against the Dark Lord. For example, he may hesitate in battles more than before.
The fact that Harry is abused gives a whole new depth to the story. It's not just about a boy joining a dark lord for no reason. His abuse makes him more susceptible to Voldemort's influence. It also adds a layer of tragedy to the 'romance' aspect. Instead of a normal, healthy relationship, it becomes a relationship born out of pain and disillusionment. Harry's abuse could lead to a cycle of him trying to find validation and love in all the wrong places, with Voldemort being that wrong place in this fan fiction.
Well, in such a fanfiction, it could completely flip their relationship. Maybe Harry realizes that Voldemort has some hidden motives that are not entirely about killing him. It might be that Voldemort sees some value in Harry that he wants to use for his own gain, rather than just destroying him like in the original story.
Harry could become more powerful in a way that scares his friends. Voldemort's training might focus on the Dark Arts, which are powerful but dangerous. This could change Harry's magic style. He may become more solitary as his new skills and knowledge set him apart from his Hogwarts friends who follow a more traditional path of magic.