They could have a mentor - mentee relationship. Gabriel, being more familiar with the Highschool DxD world, could guide Harry. For example, teach him about the different factions like the devils, angels, and fallen angels.
A friendship is very likely. They might bond over shared interests such as a love for adventure or a desire to protect the innocent. They could support each other in various school events or battles. For instance, in a school tournament, they could be each other's cheerleaders and also strategic partners.
Well, the plot could involve Harry being a new transfer student to the Highschool DxD world and Gabriel being one of the first to befriend him. Maybe they team up to face some of the common threats in that universe, like the devils or fallen angels. They could also explore their own unique powers together and learn from each other.
One possible fanfiction could be about a crossover where the characters from Highschool DxD and Naruto are transported to a world where they meet Gabriel. Maybe they team up to fight a common enemy. For example, the devils from Highschool DxD and the ninjas from Naruto could combine their powers under Gabriel's guidance to defeat an ancient evil that threatens all three of their realms.
He could still have his chakra - based powers. His Rasengan, for example, would be a powerful attack. And with his heightened senses, he might be able to detect and counter attacks more quickly. He could also develop new techniques that rely on his other senses, like a sound - based jutsu that can stun opponents.
You can try fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Highschool DxD Issei x Gabriel' on their search bars, and you should be able to find relevant fanfictions.
The fanfictions also allow for a deeper exploration of the characters' personalities. Issei's determination and Gabriel's grace can be further developed. Writers might give them new backstories or put them in situations that are not in the original anime. This gives readers a fresh perspective on the characters and their relationship, which is what makes these fanfictions so interesting.
In fanfiction, Ophis and Issei could have a mentor - mentee relationship. Ophis is extremely powerful, and she might take Issei under her wing to help him grow stronger. This could involve her teaching him about his powers and how to better control his devil abilities.
One common theme could be the relationships between the characters. Since it involves Gabriel, there might be themes of friendship or rivalry with other characters from the 'Highschool DxD' universe.
One idea could be a tournament - style plot. Characters from all three universes are invited to a grand tournament. Naruto's ninjas, the devils from Highschool DxD, and Gabriel's angelic forces compete against each other in various challenges. This would showcase the different fighting styles and powers of each group.
It could be really interesting. Maybe the magic from Harry Potter could be introduced into the world of Highschool DxD. For example, the wizards could use their spells in the battles that happen in Highschool DxD. The characters from both universes might interact in unexpected ways. Harry Potter characters could be shocked by the different types of beings in Highschool DxD, while the characters from Highschool DxD could be fascinated by the magic of Harry Potter.
Cloud has his iconic Buster Sword skills. In a Highschool DxD fanfiction, these could be translated into a very powerful physical attack ability, perhaps even on par with some of the high - level sacred gears. His speed and agility in combat would also be a great addition, allowing him to quickly move around the battlefield.