One popular pairing could be Gohan and Rias. Their personalities are quite different, with Gohan being more reserved and Rias being a strong - willed devil. The contrast between their powers and backgrounds could create an interesting dynamic in the fanfiction.
Some fanfictions might also feature Gohan with Xenovia. Since Xenovia is a strong - willed and determined character, and Gohan is also very powerful and has his own set of values. Their relationship could be explored in terms of how they grow together, both in terms of power and in understanding each other's worlds.
Ben and Issei would be an interesting pairing. Ben is used to being a hero with his Omnitrix, and Issei has his own journey of becoming a powerful devil. They could learn from each other's combat styles. Ben could show Issei how to use his alien forms' unique abilities in battle, and Issei could teach Ben about the world of devils and the importance of relationships in his world.
Gohan and Issei would be really interesting. Issei is all about being a pervert and getting stronger in his own way. Gohan is this powerful yet humble Saiyan. Their different outlooks on power and relationships would create a lot of humorous and thought - provoking moments.
Rias Gremory is also quite popular. When she is paired with Alucard in fanfiction, their different personalities and powers create interesting scenarios. She is a strong and intelligent character, and her relationship with Alucard can range from friendly cooperation to a more complex, power - based relationship.
Issei from High School DxD is often a popular character in these crossovers. His perverted yet determined nature can create some interesting interactions when combined with the Pokemon world. Maybe he tries to use his charm on female Pokemon trainers or gets excited about rare Pokemon in a way that's similar to his reactions to girls in the original series. Another popular character could be Pikachu. Pikachu's cute appearance and powerful electric attacks make it a favorite, and seeing it interact with the characters from High School DxD can be really entertaining.
Another common pairing is Sasuke and Akeno. Sasuke's brooding and powerful nature might attract Akeno, who is also quite powerful and has a bit of a mysterious side herself. They could bond over their shared power levels and a mutual understanding of what it means to be different from the norm in their respective worlds.
Akeno is another fan - favorite. Her dual nature, being both sweet and somewhat sadistic, can be explored in different ways in fanfiction. At Sans Seraphim High School, she could be involved in a love triangle or be the key to solving a mystery related to the school's dark past.
Naruto Uzumaki is often a very popular crossover character. His status as a powerful ninja and his unique personality make him stand out. Fans like to see how he would interact with the characters from High School DxD.
Rias Gremory is also highly favored. Her beauty, power, and leadership qualities make her an interesting character to write about. In an Akatsuki High School setting, she could be leading her peerage in new challenges, like dealing with rival clubs or facing off against new threats within the school.
Issei is often a popular character in such fanfictions. His original character traits from the High School DxD series combined with the new elements from the gamer cheat concept make him very interesting.
Issei from High School DxD is also popular in crossovers. His pervy nature could create some humorous situations when interacting with the more serious Naruto characters. Also, his power as a devil and his ability to boost his power through his perverted desires can be an interesting contrast to the chakra - based powers in Naruto.