Perhaps the OC is a skilled musician and Sona, who has an appreciation for the arts, meets the OC at a school concert. They start collaborating on music projects and their relationship blossoms from there. The OC could also have some sort of hidden past related to the supernatural world that Sona helps them uncover.
Perhaps Alex Black has a mysterious past that Sona is intrigued by. They could start off as acquaintances, but as time goes on, they form a deeper bond over shared interests in magic and the supernatural world of Highschool DxD.
One common theme is the clash and combination of different power systems. Naruto has his chakra - based ninja powers, while the DxD universe has its own system of magic and power levels. So, seeing how these interact is a big part of it. For example, Naruto might teach Sona some ninja techniques.
Another theme is the exploration of different worlds or abilities. Sona has her own unique powers and world in the League of Legends universe. Fanfictions often explore how the OC interacts with this world and Sona's abilities, such as in 'Into Sona's Realm' where the OC gets to experience Sona's magical realm first - hand.
Start with small interactions. For example, have them meet in the school library while looking for books on magic. This initial encounter can be the foundation for their relationship.
Well, another plot could be that the OC is an old friend of Sona from a different dimension. They are reunited in the High School DxD world. The OC has a special ability to see the future in fragments. Using this ability, they start to foresee some dangerous events that could endanger not only Sona but the entire school. So, they work together to prevent these disasters. This could involve a lot of strategy and some cool battles against various foes.
Power - up themes are also common. Naruto already has his powerful ninja abilities, and when combined with the magic and power systems in High School DxD, there are often scenarios where he or Sona gain new powers or find new ways to use their existing ones. For example, Naruto might learn to combine his chakra with Sona's magic for a super - attack. Another theme is friendship and alliance. Since they are from different worlds, they need to form an alliance to face common enemies. This often leads to the development of a deep friendship between them as they rely on each other's strengths.
Friendship is a common theme. Tsubaki and Sona often start with a pre - existing friendship in the fanfictions and it either deepens or is tested. For example, they might have to support each other through tough times at school or in the supernatural world.
A common plot is Sona's struggle for power within the Devil society. In many fanfictions, she has to face off against other powerful devils while also trying to protect her own peerage. For example, in some fics, she battles against devils who want to take over her territory.
A common theme is friendship. The oc might start as a friend to Sona Sitri and then the relationship could develop further. Another theme could be adventure, where they go on quests together in the High School DxD universe.