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.
Some characters might be immediately terrified. They could avoid Izuku or even try to drive him away. For example, classmates who don't understand his quirk might spread rumors about him.
Izuku's character development in this fanfiction could be quite complex. With a dark matter quirk, he may initially be ostracized or feared by some due to the unknown and potentially dangerous nature of his power. This could lead to him feeling isolated, but it also might fuel his determination to prove that he can be a great hero. As he trains and explores his quirk, he would learn about its limits and potential. His moral compass could also be tested, for instance, if the dark matter has some unforeseen negative effects. Through these experiences, he would gradually transform from a somewhat naive boy into a more mature and self - assured hero, all while grappling with the implications of his unique quirk.
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.
He might become more confident. Having such powerful abilities could make him feel more capable of taking on any challenge. He wouldn't be as timid as he was before.
He could become more protective. Since Kiva's powers are powerful, Naruto might feel a greater sense of responsibility to protect his village and friends. He might also be more secretive about his new powers, at least until he fully understands them. And his sense of humor might change a bit, becoming a bit darker or more sarcastic as he gets used to his new identity as someone with Kiva - like abilities.