An OC (Original Character) in a Highschool DxD fanfiction having the Rinnegan could be an extremely powerful and unique addition. The Rinnegan, from the Naruto universe, brings a whole set of abilities like the Six Paths powers. In the context of Highschool DxD, this OC could use these powers in various ways. For example, the ability to see chakra could be translated into seeing the magical energy in the DxD world. They could use the gravitational powers of the Rinnegan to manipulate the environment during battles against devils, angels, or fallen angels. It would make for a very interesting and overpowered character in the Highschool DxD fanfiction setting.
The balance can also be achieved by making the OC need to train and develop their Rinnegan powers gradually. They start with basic abilities and as the story progresses, they learn more complex and powerful ones. This way, they face challenges appropriate to their current power level at each stage of the story, and the power growth feels more natural and not overly sudden.
The OC's Rinnegan in a Highschool DxD fanfiction would be extremely powerful. It could give the character abilities like seeing through illusions, which would be very useful against devils known for their trickery. The power to summon creatures from the Six Paths could also be a game - changer. It could summon powerful beings to fight alongside the OC in battles against other supernatural entities in the DxD world.
The Rinnegan OC can stand out by having a moral code that is different from the typical characters in the gamer fanfiction. For instance, they might use their powers not just for victory but also to protect the virtual world's inhabitants in a more selfless way, which would make them a more complex and interesting character.
One idea could be an OC with the Rinnegan who has the power to manipulate game glitches in the virtual world of the fanfiction. They could use it to their advantage, like teleporting within the game or accessing hidden areas.
One idea could be an OC with the Rinnegan who is from a hidden clan. They use their Rinnegan powers to uncover ancient secrets that could change the ninja world. For example, they might find a lost jutsu that was sealed away long ago.
Give your OC a unique backstory. For example, they could have obtained the Rinnegan through a strange accident or a family curse. This will make their character more interesting.
One idea could be an OC with the Rinnegan being a transfer student to the Highschool DxD world. Their Rinnegan abilities could attract the attention of different factions. For example, they might be able to use the Preta Path to absorb the demonic energy attacks of the devils, which would both intrigue and alarm the devils in the school.
Perhaps the OC is a healer who is the only one who can keep Naruto's body stable when he overuses his Rinnegan. Their relationship starts as a necessity but then develops into a deep friendship or more. They get involved in a conflict where Naruto has to use his Rinnegan to save the OC's clan from destruction, and in the process, they learn more about the secrets of the Rinnegan and their own destinies.
The OC can be designed to be strong - willed like Naruto. Since Naruto is determined to protect his friends and the village, the OC could have a similar sense of loyalty. For example, the OC could be a ninja from another village who comes to Konoha seeking help and is inspired by Naruto's values. Their shared goal of protecting the innocent can make them a great pair.