In this fanfiction scenario, Harry Potter might change in several ways. His perspective on life and death could shift. Knowing that his brother has some sort of'reborn' aspect could make him question the finality of death more. He could also become more empathetic towards those who have lost loved ones. Emotionally, he might be more stable as he has a family connection now. This new bond could also make him more willing to take risks in the name of protecting his brother, which is a different kind of motivation compared to just fighting Voldemort for the sake of the wizarding world. Additionally, he may develop new magical abilities or skills as a result of the influence of his brother's unique situation.
In the original story, Harry often felt alone in his battles. But with an OC as his brother, there would be a constant ally. The OC could have different relationships with the teachers too. Suppose the OC has a particular interest in potions like Snape. This could lead to a different kind of mentorship and perhaps change Snape's view of the Potter family. And when it comes to the final battles, the OC could bring in additional magical artifacts or allies that were not part of the original story, completely changing the way the fight against Voldemort unfolds.
He might become more in tune with nature. Since the Phoenix is a symbol of rebirth and nature in the wizarding world, Harry could develop a stronger sense of environmental magic. For example, he could be able to communicate with other magical creatures more easily, not just the Phoenix.
He would likely become more hardened. In the Harry Potter world, he had his friends and Hogwarts as a sort of safe haven. As a Stark, he'd be in a much harsher world. He'd probably be more aggressive in protecting his family and territory.
If reborn into Marvel fanfiction, Harry Potter's character would experience significant changes. His sense of self - reliance would be tested as he'd be surrounded by powerful superheroes. He'd have to learn to rely on others more, which could make him less solitary in his decision - making. His view of power would also expand. In Hogwarts, magic was the main form of power, but in Marvel, there are so many different sources of power like super - soldier serum, gamma radiation, and alien technology. This would make him more curious and eager to learn about different forms of power and how to use them effectively. His moral compass might also shift slightly as he'd encounter different ethical dilemmas in the Marvel universe that are not the same as those in the wizarding world.
He becomes more humble. Elves are used to serving and being in a subservient position. Growing up with them, Harry would learn the value of every being, no matter their status. He'd be less likely to show off his magical abilities like he might have in the original story.
He might become more sneaky and calculating. Since he's a thief now, he would have to be good at hiding his actions and planning his 'heists'. His moral compass might also shift a bit as stealing isn't exactly an ethical behavior in the normal wizarding world.
He becomes more brooding. The knowledge of necromancy and the power it brings can isolate him as he realizes it's a power not many understand or accept. So he might withdraw from his usual jovial self.
He'd be more adaptable. Quarians live in a different way than wizards, so Harry would learn to be flexible in new situations.
It's hard to say for certain. The Harry Potter fanfiction community is vast and diverse. Some writers like to create alternative universes where characters have completely different relationships. But given the nature of Harry and Lucius in the original series - one being the hero and the other a Death Eater - making them brothers would require a great deal of imagination and explanation. The writer would need to explore how this new relationship would affect the plot, the other characters, and the overall themes of the story. It might also change the way we view the magical world's social and political structures as they are presented in the books.
Harry would probably be more attuned to dark magic. His upbringing by Satan would expose him to different forms of magic and he would have a different understanding of power. He might be more ruthless in achieving his goals and less concerned about the consequences for others. For example, in the original series, Harry was always worried about the safety of his friends, but in this fanfiction, he might use his friends as pawns if it served his purpose.