She would probably become more isolated from the good side. Her values would shift towards power and self - interest. She'd start to see the world in a more 'survival of the fittest' kind of way. She might also develop a more calculating and manipulative side. Instead of relying on friendship and loyalty like before, she'd be more likely to use people to get what she wants. For instance, she could pretend to befriend some Death Eaters just to gain more influence within Voldemort's circle.
Her character would likely become more ruthless. Since she's joining Voldemort, she'd have to adapt to his ways of using dark magic without hesitation. For example, she might no longer have the moral qualms about using the Unforgivable Curses.
Well, her character would experience a significant transformation. She'd lose her previous sense of justice and fairness. Her appearance might also change to reflect her new dark persona. Maybe she'd start wearing darker robes and have a more menacing look. Emotionally, she'd become colder and more detached. She'd view the lives of others as less important than her own goals. This could be a gradual change as she gets more involved with Voldemort's plans and the dark magic he uses. She might even start to enjoy the fear she instills in others, which is a far cry from the empathetic Harry we know from the original story.
Well, think about it. Harry's relationship with his friends is ruined. Hermione might try to convince him to come back to the good side, but Harry, feeling so betrayed by his past experiences, would resist. With Voldemort, Harry would be in a very strange position. Voldemort doesn't really trust anyone, but he would use Harry to his advantage. Harry, in turn, might start to see Voldemort as a sort of father figure, which is a very warped view considering Voldemort's nature. And the wizarding world as a whole would view Harry as a fallen hero, and there would be a lot of fear and hatred towards him.
She would become more cynical. Instead of her usual optimistic self, she would start to see the world in a more negative light. Her trust in others would be severely damaged. For example, she might not believe in the goodness of her friends anymore.
When Fem Harry joins Voldemort, it completely flips the power balance. The Ministry of Magic, which was already in a tenuous position, would be even more unstable. They would be unsure how to deal with a situation where the once - hero is now on the side of the dark lord. The Hogwarts students would be divided. Some might be inspired by Fem Harry's actions, thinking there's more to the story, while others would be heartbroken. This would also lead to a change in the way magic is perceived. Dark magic might start to seem more appealing to some, and light magic less so, creating a moral gray area in the wizarding world.
Well, it could be that her joining Voldemort causes a huge shift in the balance of power. The wizarding community would be in chaos as they try to figure out how to deal with this new threat. Dark Fem Harry might also start to influence Voldemort in unexpected ways. Maybe she convinces him to use more psychological warfare rather than just brute force. And she could be the one to find and destroy some of the remaining Horcruxes of other wizards, believing that it would make Voldemort even more powerful and thus her position more secure.
At first, Harry might be hesitant. But as he spends more time with Voldemort, he becomes more hardened. His values start to shift. For example, he may no longer see killing as an absolute wrong if it serves his new goals. His relationships with his old friends like Hermione and Ron will be severely strained. He'll become more secretive and calculating.
In these fanfictions, Harry's character transformation is quite drastic. He loses his sense of morality. His loyalty, which was once towards his friends and the good side, now shifts to Voldemort. He becomes more power - hungry, willing to do whatever it takes to gain more power, including using dark magic without hesitation. He also becomes more secretive and manipulative, using his former relationships to further Voldemort's goals.
She becomes more confident. In these fanfictions, as fem Harry takes on the Dark Lady role, she no longer doubts herself like she used to. For example, in many stories, she used to be shy about using her magic, but as the Dark Lady, she wields it with great confidence.
Her character would likely become more regal and composed. In the role of Queen of England, Fem Harry Potter would have to represent the nation. So, she'd need to be more polished in her speech and mannerisms. She would also be more calculating in her decision - making, thinking about the long - term effects on the country rather than just her immediate circle. Also, she might be more secretive about her magic, using it only when necessary to avoid causing panic among the non - magical population.
She would become more cynical. Instead of seeing the good in people, she would start suspecting everyone's motives. For example, she would no longer trust her former friends easily.
Her life would change drastically. She'd be living in luxury and surrounded by all the trappings of royalty. She'd have to learn proper etiquette and protocol. For example, she'd have to know how to curtsey and address different members of the royal family correctly.