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 an enemy who discovers Stiles' secret. They might be looking for any weakness in Stiles and figure out that his secret identity as a girl could be used against him. They could then try to expose Stiles to cause chaos and harm to him and his friends.
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.
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.
Another character he could interact with is Thor. Thor's otherworldly nature and his powerful abilities would be quite a contrast to Stiles. Maybe Stiles could help Thor understand some earthly concepts while Thor could teach Stiles about Asgardian culture and power.
It could be an original character created just for this fanfiction. Maybe a girl who Stiles met during a summer vacation and had a brief fling with.
It could be his love interest, like I mentioned before, Lydia. She might reject him because she's too focused on her own goals or perhaps she doesn't see Stiles in that way at the time. Their relationship has always been a bit complex in the show, so this would be a likely scenario in fanfiction.
A kind stranger could come to his aid. Maybe someone who has been through a similar situation in the past. This person could offer practical advice on how to survive while being on the run and pregnant. They might also teach Stiles some skills like cooking healthy meals for the baby's sake.
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.
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.
In this fanfiction, Stiles and Jackson are central. Stiles, with his sarcastic humor and loyalty to his friends, and Jackson, who is trying to find his place not just in the social hierarchy but also within the group of friends. Their relationship, whether it's a newfound friendship or a more complex dynamic, is what drives the story forward.
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.