Nagisa Shiota as a vampire could develop a more brooding and mysterious personality. He would be constantly aware of his difference from humans. However, his underlying kindness might still shine through. For example, he might only feed on animals instead of humans, which shows that his core values haven't completely changed. His relationships would also be affected. He might be more hesitant to get close to people for fear of his vampire secret being exposed.
If Nagisa Shiota becomes a vampire in fanfiction, his personality would likely have a complex transformation. Initially, he'd be in shock and confusion over his new state. This could lead to mood swings. As he adapts, he may become more calculating, as vampires often need to be strategic about their actions. But his loyalty to his friends would remain strong. He'd be more protective, using his vampire abilities to safeguard them. He might also develop a new sense of independence, as he now has powers that set him apart from the normal human society he used to be a part of.
He might become more withdrawn at first. The knowledge of his new, blood - thirsty nature could make him afraid of hurting those he cares about. But over time, he could become more confident in controlling his vampire side and use his newfound power to be more assertive.