If Naruto were Rias's father, his relationship with the other characters would be full of surprises. He'd likely be very protective of Rias's friends in the Occult Research Club. With the devils in the Gremory family, he'd bring in a new dynamic. His straightforward and often naive - seeming nature (but actually very wise) would both amuse and confuse them. He'd also have an impact on how Rias interacts with her peerage. His ideas about friendship and loyalty could seep into the way she leads her group, and this would in turn affect how the other characters in the DxD world view her and her group.
Naruto would have a very unique relationship with the characters. With the devils in general, he'd be an outsider with his ninja powers. But because he's Rias's father, they'd have to respect him. He might have a friendly rivalry with some of the stronger devils, showing them his ninja skills. For the angels, he'd be a wildcard. They might be suspicious of him at first, but Naruto's good - natured personality could win them over. And with the fallen angels, he could be a mediator of sorts, using his experience with different factions in his own world to try and keep the peace in the DxD world.
He'd probably have a complex relationship with Sirzechs. As Rias's father, Naruto would be protective of his daughter and might be cautious around Sirzechs at first. But given Naruto's ability to see the good in people, they could end up as allies. With Issei, Naruto might see a bit of himself in the young pervert - turned - hero and could offer him some guidance in terms of power and relationships.
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.
There could be a storyline where Naruto is summoned to the High School DxD world by some ancient magic. He is seen as a powerful ally or a threat depending on who you ask. Rias takes it upon herself to figure out Naruto's intentions. As she gets to know him, she realizes that his values of friendship and protecting the weak are similar to her own. Together, they start a journey to stop a dark force that is rising in the DxD world, with Naruto teaching the devils some ninja techniques along the way.
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.
He would still likely have his chakra - based powers from the Naruto universe. So he could use techniques like the Rasengan or Shadow Clone Jutsu. However, due to his emotionless state, he might use them in a more precise and less flashy way. His Nine - Tails chakra could also be present, but perhaps he would have better control over it since emotions often cloud his control in the original series.
Naruto is very outgoing and has a big heart. He's always looking to make friends and help others. Rias is a strong and confident leader. Naruto's friendliness could soften Rias's more serious side. Rias, in turn, could help Naruto better understand leadership and strategy, which he might lack a bit in his own world.
Evil Naruto would still have his basic ninja powers. He'd have his shadow clones which could be used for various purposes, like spying or overwhelming opponents in battle. His Rasengan would also be a very powerful attack. Additionally, he could have some dark - themed powers due to his evil nature. Maybe he could corrupt his chakra to have a more sinister effect on his enemies, like causing them to feel despair or weakening their will.
Perhaps they are hiding because they have stumbled upon a secret that many powerful beings in both universes don't want them to know. While under Rias, they start to figure out the mystery. Kiba might use his knowledge of chakra and ninja techniques to keep their hiding place concealed, and Issei could use his connection with the devils to gather some inside information. Along the way, they might make new friends or enemies from both Naruto and High School DxD worlds.
His chakra abilities might be affected by the magic in the DxD world. Maybe his Rasengan could gain some magical properties.
Dark Naruto in a High School DxD fanfiction would be a complex character. He would carry the trauma and power-hungry tendencies from his Naruto backstory. In the DxD world, this could manifest as a drive to become stronger, not just physically but also in terms of influence. He might be initially distrusted due to his dark nature, but his loyalty and determination could win over some of the characters. He could be a wildcard, not fully aligning with the devils, angels, or the other factions, and always looking out for his own interests while also having a sense of justice that is unique to him.
Rias' demonic powers, like her ability to use destruction magic, could be used to support Naruto in battles. For example, if Naruto is facing a large group of enemies, Rias could use her magic to clear a path or take out some of the stronger foes. Also, her teleportation ability would be great for getting them out of tight spots quickly. Her leadership skills as a high - ranking devil could also help Naruto in strategizing during battles or when dealing with political situations in the underworld.