His personality could be split. On one hand, he would still have his core values of heroism and kindness. But on the other hand, the dark quirk could bring out a more ruthless side. He might find himself having internal debates between using his power in a traditional heroic way or succumbing to the darker impulses that the quirk brings. This internal struggle would be a major part of his new personality in the fanfiction.
He might become more confident. Having a powerful dragon quirk could make Izuku realize his own strength, and he'd carry himself with more assurance. For example, he wouldn't be as shy when speaking up in class or to other heroes.
His relationship with Bakugo might change. Bakugo could be jealous of Izuku's unique quirk at first. But later, they might have to team up for an underwater mission. Izuku's friends from school could also be in awe of his mermaid quirk and might want to learn more about it.
Naruto might become a bit more confident and cocky. Shego is often shown as self - assured, and having her powers could make Naruto feel more powerful. He might start showing off a bit more during battles.
In a fanfiction with a Dark Souls - inspired quirk for Izuku Midoriya, his character development would be a journey of self - discovery. For instance, if his quirk was 'Grim Resolve', which gives him a boost in strength and speed when he's in a near - death situation (similar to the 'hollowing' concept in Dark Souls), he would have to learn when to push himself to that limit. He would start off as a naive and somewhat idealistic character, but as he experiences the harshness of his quirk and the battles he has to fight, he would become more cynical. But at the same time, he would also find a deeper sense of purpose. His interactions with his classmates and mentors would change as they try to understand and help him with his unique quirk, and he would grow into a more complex and mature character as a result.
His relationships with his friends would likely become more complex. If he starts using his time quirk to change things, his friends might not remember the original timeline, but Izuku would. This could create a sense of isolation for him as he holds memories that no one else does.
One idea for Izuku Midoriya's quirk in a Dark Souls - inspired fanfiction could be 'Soul Link'. This quirk would enable him to form a link with other people's souls. He could use this link to either heal them by sharing his own energy (similar to Estus Flasks in Dark Souls), or he could disrupt their soul's energy, causing them to be weakened or even stunned. It could also work as a way for him to sense the presence and intentions of others, as he would be attuned to their souls. This quirk would give him a unique advantage in battles and also in forming relationships with other characters in the fanfiction.
Naruto's personality might still shine through even as an Arrancar. He's always been determined and kind - hearted. However, he could become a bit more solitary at first as he adjusts to his new form and powers. The Arrancar nature might make him more cautious in some ways, but his basic values of friendship and protecting the weak would remain. He might also develop a bit of an edge, as Arrancars are often seen as more menacing, but it would be a surface change. Deep down, he'd still be the Naruto who wants to bring peace and joy to his village.
The Gamer Quirk also changes Izuku's relationship with others. He can now offer unique insights based on his quirk. For instance, he might be able to analyze a villain's attack pattern like it was a boss fight in a game. His friends and allies can then use this information to their advantage. This makes him a valuable member of any team. At the same time, it also means that he might be seen as a bit of an oddball because his thinking is so different from the norm. But overall, it helps him form deeper connections with those who understand and appreciate his quirk.
Harry's personality would be greatly influenced. He would be more polished in his behavior, as Narcissa would demand propriety. He might be less likely to take risks without thinking, as Narcissa would have instilled in him a sense of caution. His sense of loyalty would be more complex. Instead of the straightforward loyalty he has in the original story, it could be divided between Narcissa's family values and his own sense of right and wrong. Also, he may have a more aristocratic air about him, with a refined way of speaking and interacting with others.
Naruto's personality would be a complex mix. On one hand, he might become more reserved due to Vader's influence. But on the other hand, his natural inclination towards goodness might cause him to constantly struggle against Vader's darker teachings. He could be a much more conflicted individual, with his usually open - hearted and friendly personality warped by the dark environment he was raised in. He might be more secretive and less trusting of others as well.