Harry would probably become more conflicted. His views on good and evil would be severely tested. Hermione might become more cunning as she tries to outwit Voldemort while living under his roof. Voldemort himself might develop a strange sort of attachment to them, which could make his actions even more unpredictable.
At first, it would probably be very tense. Harry would be suspicious of Voldemort's motives, and Voldemort would be cautious of Harry's potential to rebel. But as time goes on, they might develop a strange sort of understanding. Harry could start to see Voldemort as more than just a pure evil figure, and Voldemort might start to view Harry as something more than an enemy.
In such a fanfic, it would be a very strange and dark twist. Voldemort might use Harry and Hermione to gain more power in a different way. Maybe he wants to corrupt them and turn them against Dumbledore and the Order. He could also try to use their knowledge, especially Hermione's vast magical knowledge, for his own evil plans.
At first, it would be really rocky. Harry has a history of distrusting Snape, and Hermione might be a bit cautious too. But as time passes, Snape's care for them in his own strict way would start to show. He'd probably be tough on them about their studies, but they'd see it's because he wants them to succeed.
Often, Voldemort starts as this purely evil figure. But when he adopts Harry, he begins to show some unexpected softness. For example, he might start to protect Harry from other threats, which is very different from his usual self.
Harry might become more conflicted. He'd be exposed to Grindelwald's ideas which could make him question the black - and - white view of good and evil he had before. His confidence could waver as he's no longer in the familiar Hogwarts environment.
If Voldemort was already dead in the fanfic, the characters would develop in very interesting ways. For instance, Neville would not have to find his courage in the face of Voldemort. He could develop his skills in Herbology more freely and perhaps become a renowned expert in the field much earlier. Luna Lovegood could explore her unique brand of magic and beliefs without the overshadowing of the Dark Lord. The Weasley family would be more at ease, and Fred and George could focus on their joke shop business without the fear of Voldemort - influenced regulations or attacks.
Well, for Harry, he'd develop a deeper understanding of different kinds of magic and people. Since Tsuna comes from a different background, Harry would have to adapt his teaching methods. This would make him more flexible. Tsuna would start to develop a more adventurous spirit. He'd be more willing to take risks as he sees Harry's courage. Also, in terms of their relationship, they'd go from being strangers to a family - like unit. Tsuna would look up to Harry, and Harry would feel a need to protect Tsuna at all costs. This would shape their personalities in different ways, with Harry becoming more protective and Tsuna more trusting.
Hermione's character would likely become more complex. She might start off as a broken and fearful person due to the abuse. But with Voldemort's influence, she could gain a new sense of power and confidence. Maybe she would become bolder in her use of magic, not just relying on traditional spells but exploring darker or more unorthodox magic under Voldemort's guidance.
If they joined Voldemort, Harry might lose some of his natural courage. He'd be in a world where his usual values are flipped, and this could make him seem more hesitant. Hermione would probably have to hide her intelligence more. Voldemort doesn't like those who are too smart or independent. So she might act more subservient in public, but in private, she'd be devising plans to break free and expose Voldemort's true nature.
In this fanfiction, the relationship between Harry and Voldemort would be full of contradictions. Voldemort adopts Harry for his own purposes, but Harry has his own will. In the beginning, Harry may be forced to follow Voldemort's orders out of fear. As time passes, he might start to question Voldemort's actions and ideology. Voldemort, seeing Harry's growing independence, could become more controlling. This could lead to a cycle of rebellion and suppression in their relationship. Harry may also find some common ground with Voldemort in their shared loneliness or desire for power, which would add more layers to their complex relationship.