Perhaps it's a new character introduced just for the fanfiction. A mysterious stranger who Stiles meets by chance and falls deeply in love with. They get married in secret because they want to get to know each other better without the interference of others first.
It could be Lydia. They have a long - standing relationship in the show, and a secret marriage between them would be quite a shock. Maybe they eloped in a moment of passion and are now trying to figure out how to break the news to their friends.
In some fanfictions, his secret spouse could be an original character created by the author. This character might be a fellow wizard or witch who has a special connection with Snape, perhaps they met during his early years at Hogwarts or in some other part of the wizarding world. They could share a common interest in potions or some other magical art, which brought them closer.
Lucy could be a potential spouse. They have a very close relationship in the original story. Maybe they got secretly married in a moment of intense emotion. It would be a big shock to their friends, but also a very interesting development as they already have such a strong bond.
His best friend could discover it. Since they are so close, they might notice something different about Stiles that makes them suspect he's actually a girl.
It could be a reformed villain. Maybe someone like Grant Ward who had a complex relationship with Skye. If he had somehow redeemed himself off - screen and they fell in love, Skye would have to keep it a secret because of Ward's past actions against S.H.I.E.L.D.
A plotline might be that Stiles got secretly married while on a trip away from his friends. When he returns, he has to figure out how to integrate his new married life into his old friendships. There could be comical moments like trying to hide his wedding ring or coming up with excuses for why he can't hang out at certain times. Also, the relationship within the marriage could be explored. Is it a marriage of convenience that slowly turns into love? Or was it a passionate love - at - first - sight kind of marriage that they now have to deal with in secret.
Her spouse could be a nature spirit. Since Artemis is closely associated with nature, a nature spirit would be a fitting partner. They could understand her love for the forest and all living things.
It could be Draco Malfoy. In some fanfictions, the dynamic between a female Harry Potter and Draco is reimagined, and they might end up secretly married as a way to show a new kind of relationship between the two formerly antagonistic characters.
Maybe some of the Akatsuki members could be his secret allies. For example, Itachi might have had a secret plan and saw Naruto's potential, so he was secretly helping Naruto become stronger all along. Also, there could be some minor characters from other villages who have recognized Naruto's hidden power and decided to ally with him.
Apollo could marry Artemis' handmaiden. Since he often crosses paths with Artemis and her group, he might fall for one of the gentle and nature - loving handmaidens. This would be an interesting choice as it would also create a connection between Apollo and Artemis in a new way.
A witch could be an interesting match. Given Elijah's world of vampires, a witch spouse could bring in a whole new dynamic. The witch could use magic to help protect Elijah and their relationship from other threats in the supernatural world. They could also have a power struggle at times, which would add spice to the relationship.