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.
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.
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.
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 singing ability would likely change his relationships in multiple ways. For the devils in his peerage, it could be a source of pride as he becomes known for this unique talent. They might promote his singing, which would increase his visibility in the supernatural community. For the female characters, it could be a romantic draw. His singing could be a way for him to express his emotions in a more nuanced way than just through his actions in battles. For those who are initially skeptical of Issei, like some of the more traditional devils, his singing could be a way to break down barriers and gain their respect as they see him as more than just a typical reincarnated devil.
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.
Gohan's super strength would make him a force to be reckoned with. He could easily overpower most of the regular characters in High School DxD. His ki blasts could also be a new form of attack that the others in the universe have never seen before.