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.
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.
One possible fanfiction could be about Rias and Sona teaming up for a school event. They might put aside their usual competitiveness and work together to organize a devil - related festival at the school. They could combine their unique skills and powers to create an amazing event that not only impresses the other students but also strengthens their bond as friends.
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.
The unique character of Alex Black. Since he's not an original character from Highschool DxD, the fanfiction can take him in all sorts of new directions within the established world of the series.
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.
Explore their differences and how they deal with them. Sona is a devil with her own set of values and powers. The OC could be from a different background, maybe a human with some latent supernatural abilities. Their different perspectives can cause friction at first but as they learn to understand each other, it can turn into mutual respect and love. For instance, Sona might be used to a more hierarchical way of doing things in the devil world, while the OC might have a more egalitarian view. By working through these differences, their relationship can develop in an interesting way.
First, think about their backstory. For example, if they are a devil, what made them become one? Maybe they had a difficult family situation in the devil world. Second, give them unique abilities that fit within the High School DxD universe. It could be a special type of magic or a physical ability like super speed. Also, consider their personality. Are they shy, bold, or mischievous? And finally, think about their relationships. How do they interact with Sona? Do they admire her, or are they rivals? All these elements combined will make a believable character.
One way is to explore her family background more. For example, showing how her upbringing in the devil society influenced her leadership style in the student council.
A fanfiction could show how Rias and Sona first became friends. Maybe it was during a difficult time at school when they both had to face some sort of discrimination from other devils because of their different leadership styles. They found comfort in each other and decided to support one another from that moment on.