He could develop a darker side to his personality. Learning from Voldemort, he may become more ruthless in achieving his goals, even if those goals are still ultimately good. For instance, he might be more willing to use unethical means to defeat other dark wizards. But at the same time, he would also be constantly struggling against this new - found darkness within him. He would be in a battle not just with external enemies but also with his own changing self.
Harry may also develop a greater sense of responsibility. Knowing that Dumbledore has chosen him as an apprentice implies a certain level of trust and importance. He would feel the need to live up to Dumbledore's expectations and use his new - found knowledge and skills to protect the wizarding world. He could start to take on more leadership roles and be more proactive in the fight against the dark forces.
Harry's character would likely be a mix of confusion and determination. He'd be confused by Voldemort's teachings which go against his natural inclination towards good. But his determination to find his true self would remain. He might develop a sort of duality, acting one way around Voldemort and his followers, but secretly being someone else.
Harry's character would likely become more complex. Initially, he might be distrustful of Voldemort's motives but also feel a sense of obligation for being saved. This could lead to him being more cautious and calculating in his actions, as he has to navigate the dangerous waters of being associated with the Dark Lord.
At first, Harry would be conflicted. He has always been against Voldemort, but now being forced to be with him would make him question his own beliefs. His character could become more cynical as he is exposed to Voldemort's dark world.
In such a fanfiction, Harry would grow in his understanding of the dark side of magic. Being captured by Voldemort means being exposed to the darkest and most malevolent forms of magic. This would make him more cautious in his own use of magic, but also more knowledgeable about how to counter dark magic. His character would also become more patient. He can't rush into things as he is in a very dangerous situation. He has to bide his time, wait for the right moment to escape or fight back. This new - found patience would be a valuable asset in his future battles against Voldemort. Moreover, he would develop a stronger sense of self - identity. He would be constantly reminded of who he is and what he stands for in the face of Voldemort's attempts to break him.
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 would be more open to dark magic. Since Voldemort is his adoptive father, he would be exposed to different aspects of magic earlier. He might not have the same fear of the dark side as in the original story. For example, he could be more confident in using spells that are considered dark but for good reasons, like using the Imperius Curse to stop a criminal.
In such a fanfiction, Voldemort would view Harry as his possession. He'd order Harry around, thinking he can use Harry's wolf - like abilities for his own dark deeds. However, Harry would not go down without a fight. He'd constantly be trying to undermine Voldemort from within. Their relationship would be a very strange and twisted one. Harry would have to pretend to be loyal while actually hating Voldemort with every fiber of his being. And Voldemort, being so arrogant, might not realize how much of a threat Harry still is even in his wolf form until it's too late.
Harry's character might become more empathetic. Since he's in this strange father - son relationship with Voldemort, he has to try to understand Voldemort's motives. This could make him less quick to judge and more willing to look for the good in others.
Well, he might become more confident and self - assured. After all, defeating the most powerful dark wizard of all time is no small feat. He could also be a bit more somber, having seen the true horrors of the dark side up close.