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.
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 plot could also be centered on a mystery related to their family. The brother of Rias discovers some secrets about their family's past that put them both in danger. He then has to work with Rias to uncover the truth and stop whoever is trying to use their family secrets against them. This could involve battles, political intrigue within the Devil world, and self - discovery for both of them.
Naruto might teach some of his ninja skills to the students in the High School DxD. For example, his tree - climbing and water - walking techniques could be really interesting for them to learn. Rias could be intrigued by Naruto's determination and his never - give - up attitude. She might see him as a valuable addition to her peerage. And in return, Naruto could be fascinated by Rias's devil powers and the way she manages her group. They could also have some cultural clashes, like Naruto's more simple and straightforward ninja culture compared to the complex and often flamboyant devil culture in High School DxD.
Well, if Naruto were Rias' twin brother, it would completely change both storylines. In Naruto's world, his new family connection to the Gremory family in High School DxD might introduce devilish powers and politics into the Hidden Leaf Village. This could lead to new alliances and rivalries. For example, the Akatsuki might view Naruto differently, seeing him as a potential link to a powerful devil family. In High School DxD, the devils might be more involved in the affairs of the ninja world, perhaps trying to gain access to unique ninja techniques. It would also change Naruto's and Rias' characters. Naruto might grow up with a different sense of self, having a devil heritage, and Rias would have a strong connection to the ninja world through her brother.
His character can be developed by giving him challenges. For example, he could be put in situations where he has to choose between loyalty to his family and what he believes is right. This would force him to grow morally. Also, having him face opponents stronger than him in battles would make him train harder and develop his combat skills.
One possible reason for such a fanfiction could be to explore a different dynamic in the story. Maybe the author wants to show a more complex relationship where Issei has some deep - seated issues with Rias' leadership style or her decisions. It could also be a way to create conflict and drama, which are essential elements in fan - created stories.
One possible idea for this fanfiction could be Naruto being transported to the world of High School DxD. He might end up attending the same school as Rias. Given Naruto's strong sense of justice and his combat skills, he could quickly become involved in the supernatural battles and power struggles in that world. He might also form an interesting relationship with Rias, perhaps starting as friends and gradually developing into something more as they face various challenges together.
His relationship with Annabeth would definitely be affected. She might be a bit wary at first, since Nyx is such an unknown and powerful entity. But she would also be there to support Percy as he figures things out.
Among the demigods at Camp Half - Blood, some of the more romantic - minded ones might flock to Percy, thinking he can give them advice on love since he's related to Aphrodite. But Percy would be really out of his element here. And his relationship with the more 'tough' or battle - focused demigods might be strained, as they might see his Aphrodite connection as a sign of weakness or something that doesn't fit with their warrior - like image of him.
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.