There's also the possibility of pairing Naruko with Sirzechs. His powerful and noble nature could contrast well with Naruko's more brash and energetic personality. It could lead to a story where they have to work together to solve a major problem in the DxD world, and in the process, develop a deep connection.
Sasuke is often paired with Naruko in these fanfictions. Their relationship can be explored in different ways, like they might start as rivals in this alternate universe too, but their bond develops into something more romantic or a deeper friendship. Another common pairing is with Kakashi. He could be her mentor in a more intense and personal way, guiding her not just in ninja skills but also in understanding the complex politics of the ninja world.
Issei Hyoudou is a very popular character to focus on. He's the main character in the original series and has a lot of room for development in fanfiction. Rias Gremory is also a great choice. She's a powerful devil and her relationships with other characters can be explored further. Akeno Himejima is another popular one. Her mysterious past and her powers make her an interesting character to write about.
Her friends are likely to be part of it. A close female friend could be like a pregnancy buddy, sharing her own experiences if she has any. There might also be a mentor - like figure, an older woman who has been through pregnancy and can give Naruko valuable advice on everything from childbirth to raising a child.
The father of the baby would be a key character. It could be a fellow ninja or someone from her past.
Sakura could be a potential character. She has a long - standing relationship with Naruto, and in this harem scenario, she might be jealous of Naruko at first but then come to accept the situation. Hinata could also be part of it. Given her love for Naruto, she would be an interesting addition to the harem, adding a shy and gentle dynamic.
Well, you could write a fanfiction about Naruko's arrival in the DxD world and how she adjusts to the different culture and rules there. It could be really fun to see her reactions to things like the Rating Games or the different devil clans. Or perhaps a story where Naruko tries to teach some of her ninja techniques to the DxD characters, and they in turn teach her some of their magic.
Kakashi could also be a character in 'naruto and naruko harem fanfiction'. He can be the mentor figure, guiding Naruto and Naruko through different situations. His wisdom can influence how the harem relationships develop, for example, by giving Naruto advice on how to handle multiple relationships in a proper way.
Naruko in this fanfiction is most likely a character that the author has developed for the story. It might be inspired by other characters in the genre or it could be a completely new concept. Maybe she has certain unique abilities or traits that make her the target of being double teamed. For example, if it's in a fantasy setting, she could possess a powerful magic that others want to suppress by ganging up on her.
Naruko might also gain some vampire - like powers. For example, she could have increased speed and strength. However, she would also have to face the negative aspects. Vampires are often hunted or feared in many fanfictions. So she would have to constantly be on the lookout for those who would do her harm, and she would have to learn how to control her new vampire nature so as not to attract unwanted attention.
It could be Orochimaru. He's always been interested in obtaining powerful individuals for his experiments, and Naruko with her unique abilities would be a prime target.