The main characters are Yuma and Dark Mist. Yuma is likely to have certain unique abilities or characteristics that make them an interesting protagonist. Dark Mist, on the other hand, probably has an air of mystery about it, with powers that are either dark - in the sense of being related to shadows or the unknown - or powers that are misunderstood as being dark.
Fanfiction.net is also a popular place to look for yuma x dark astral fanfiction. Just use the search bar and type in the relevant keywords. However, be aware that the quality of stories can vary widely on this site.
There could be a fanfiction where yuma discovers a hidden side of dark astral. In this story, it delves into their relationship development as yuma tries to understand dark astral better. This can include moments of trust - building and shared secrets.
One possible fanfiction could be about Yuma and Dark Mist forming an unexpected alliance. Maybe Dark Mist has some hidden motives that initially seem malicious but as the story progresses, it turns out he's actually trying to protect Yuma from a greater evil in the Zexal world.
Yuma and Tori could be characters from a small - scale, indie story that has a dedicated but not widely - known fan base. The fanfiction allows fans to expand on their relationship, imagining their married life in ways that the original story might not have delved into deeply, perhaps adding more depth to their characters and their connection.
One idea could be that Yuma and Dark Mist are from different realms and they accidentally meet. They initially have a misunderstanding but as they get to know each other, they find they have a common goal of protecting a certain magical artifact. Another could be that Dark Mist is being hunted and Yuma offers protection in exchange for knowledge about a mysterious power.