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.
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.
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.
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.
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.
To keep their marriage a secret, they would have had to be very careful. They could have met in secluded places like an abandoned forest or a hidden cave. When communicating with others, they would be extra cautious not to let any hints slip. Natsu might have pretended to be just a friend to Yukino when around others, and Yukino would do the same. They might also have had a secret code or signal to let each other know when it was safe to be together.
Well, it could be that Stiles is secretly a girl and in the fanfiction, this secret is slowly revealed as he falls in love with someone. And the person he loves starts to notice little things that don't quite add up with his supposed male identity, like his more delicate features or his different style of communication.
Some of his friends might be really surprised. Scott, for example, might be a bit worried that Stiles' vampire obsession could lead him into dangerous situations. Lydia, on the other hand, could be more intrigued and might want to know more about the fanfiction. She might see it as an interesting intellectual pursuit.
One possible plot twist could be that Stiles has been a werewolf all along but has been using some sort of magic or potion to suppress his werewolf side. This could be revealed when he is put in a life - or - death situation and his true nature emerges to save the day.
One common theme is the struggle to maintain a normal life. Stiles often has to deal with hiding his fame from those closest to him, like his friends and family. Another theme is the allure of the fame itself. Sometimes he's tempted by the perks of being famous, but also realizes the downsides.