When it comes to Yuu's relationships in this fanfiction scenario, it's important to consider how his new nature affects his emotional bonds. For instance, his feelings towards the vampire progenitor might shift. He could start to see them in a different light, perhaps understanding their motives a bit more. And his relationships within the vampire society he now belongs to would be full of challenges. He would need to learn the rules and hierarchies, and this could either strengthen or strain his relationships with other vampires, all while still trying to hold on to his human - like values and relationships with his human friends.
His relationship with Mika would be very different. Since Mika is already a vampire, they might have a new understanding of each other. However, there could also be some jealousy or power struggle as Yuu adjusts to his new vampire status. With his human friends, like Shinoa and the others, they might be initially afraid of him. But Yuu would probably try his best to show them that he's still the same person who wants to protect them.
His relationship with his human friends would definitely change. They might be scared of him at first, especially if he shows signs of his new vampire nature like a craving for blood. Some friends might try to distance themselves from him, while others might be more understanding and try to help him control his new urges.
One possible plotline could be Yuu struggling with his new vampire nature while still trying to protect his friends. He might have to deal with the thirst for blood and resist the urge to harm his comrades. Another could be that he uses his new vampire abilities to fight against the real enemies of the series in a more powerful way. For example, he could use enhanced speed and strength to take on the vampires that are causing chaos in the world of Owari no Seraph.
In fanfiction, Yuu's character might be developed by showing his vulnerability more when it comes to Shinoa. For example, he could be more open about his fears and insecurities around her. Shinoa, on the other hand, could be developed by showing her softer side towards Yuu. She might let down her tough - girl facade and be more affectionate.
In Owari no Seraph fanfiction, Mika and Yuu's relationship is complex and deep. They have a strong bond that has been through many trials. Mika is often very protective of Yuu, and Yuu in turn cares a great deal about Mika. Their relationship can be seen as a mix of brotherly love and something more profound, which is often explored in different ways in fanfiction.
One popular theme is the exploration of their friendship in extreme situations. How they support each other when facing powerful enemies in the Owari no Seraph universe.
Love and friendship are also common themes. The female Yuu may form new friendships or fall in love in ways that are different from the male Yuu. These relationships can be a driving force in the story, either helping her grow as a character or presenting new challenges. Additionally, themes of survival in the harsh world of Owari no Seraph are present, with the female Yuu having to use her wits and strength to stay alive.
You can try fanfiction websites like Archive of Our Own (AO3). There are usually a lot of works related to Yuu and Shinoa from Owari no Seraph there. Just search for their names on the site.
His relationship with Sakura would be very different. Instead of the old friendship/romantic tension, Sakura might be initially afraid of him as a vampire. She would have to overcome her fear and see that Naruto is still the same person inside. His relationship with Sasuke could also be strained. Sasuke might see Naruto's transformation as a sign of weakness or a new threat.
One great Vampire Yuu fanfiction could be 'Yuu's Hidden Powers'. In this fic, Yuu discovers new and unexpected abilities that set him apart from other vampires. It delves deep into his character, exploring how he copes with these new powers while still maintaining his core values.
His relationship with Sasuke could become more competitive in a different way. Instead of just competing in ninjutsu, they would now compete in swordsmanship skills. This could add a new layer to their rivalry.