Some characters might be afraid of the OC. If he's Satan's son, there's an immediate perception of danger. Others could be drawn to him out of curiosity or a desire to gain power through association. For example, some devils might see him as a potential ally in power struggles within the devil realm.
The relationships would be complex. Angels and fallen angels might see the OC as an enemy by default because of his relation to Satan. But among the devils, there could be a mix of envy and respect. His classmates at school could be divided between those who want to befriend him for his power and those who avoid him for the same reason. It also depends on how the OC behaves. If he's kind - despite his heritage - some might befriend him regardless. If he's cruel, it would reinforce the negative views others have of him.
Well, in some High School DxD fanfictions, the OC might be considered Satan's son to create a really powerful and unique character. It could be for the sake of having a character with a dark and mysterious origin that can bring in a lot of interesting plot twists. Maybe the author wants to explore the idea of a character with demonic powers and a connection to the most powerful figure in the underworld, Satan.
Regarding his relationship with the angels and fallen angels, it would be quite tense. As a devil and related to Rias, he would be on the opposite side in the grand scheme of things. However, there could be individual angels or fallen angels who see him as an exception. Maybe an angel who believes in the power of redemption might try to convince him to turn away from the devil ways. Or a fallen angel might see him as a potential partner in some sort of scheme against the more powerful devils. His relationships would be full of drama, suspicion, and the potential for growth and change.
Naruto's power of chakra would be a huge game - changer. His ability to perform ninjutsu like Rasengan or his more powerful attacks could easily overpower many of the initial opponents in the High School DxD world. For example, in a fight against low - level devils or fallen angels, his attacks would be very effective.
His relationship with Sasuke might become more complicated. Sasuke, who has his own dark past, might view Naruto's new identity with suspicion or envy. They could have more intense battles as Sasuke might see Naruto as a greater threat now.
He might have the ability to control some form of divine light, which could be used for both offensive and defensive purposes. This could be different from other characters' abilities in High School DxD as it would have a more divine origin.
The fairy OC would likely have different powers. Most characters in Highschool DxD have demonic or angelic powers, while a fairy would have more nature - based and magical abilities like flight, talking to animals, and using plants as weapons or shields.
His relationship with Rias would likely become more complex. She might see him in a new light, both as a powerful being in his own right and also as someone with a great destiny. There could be some tension as she tries to figure out how to interact with him now.
His ability to learn dragon languages and gain new shouts over time would mean that as the story progresses in the fanfiction, his power would keep growing. In High School DxD, the power is often based on birthright or contracts. The Dragonborn's self - made power through his efforts would be a new concept. His dragon - related abilities such as dragon riding could also give him an advantage in mobility, allowing him to quickly move between different areas in the High School DxD world, which could be crucial in battles or strategic maneuvers.
Naruto's God Slayer abilities could completely shift the power balance. For instance, if he has the power to slay gods, he could pose a serious threat to the devils' hierarchy which has some connection to the divine. This would likely lead to various factions either trying to recruit him or eliminate him.
Naruto could be seen as a wild card. His ninja abilities would be something unheard of in that world. He might start by getting into trouble because he doesn't understand all the rules. Kushina, on the other hand, would be the voice of reason. She would help Naruto navigate the new social and power dynamics, and also show the importance of family values in a world full of chaos.