There's a chance it could be a secret society. They might have some ancient prophecy or grudge related to Serena. So they capture her to fulfill their own motives, perhaps to prevent her from achieving something important in the story.
It could be a rival group. They might be jealous of Serena's abilities or resources and capture her to gain an advantage.
In this fanfiction, perhaps Serena is captured by an evil organization. They might be after her special powers or knowledge. Then, her friends would start a rescue mission.
Maybe a rogue ninja group that wants to sell Naruto to the highest bidder. They see him as a valuable asset due to his strength and status in Konoha.
Maybe it's an old foe who has come back with a vengeance. For example, a gem that Steven thought he had defeated before might return and capture him in order to get revenge. This foe could have some new abilities or powers that make it difficult for Steven to escape.
Ash could be the cause. He might accidentally break a promise to Serena, like promising to watch her perform in a Pokemon Showcase but then missing it because he got caught up in a Pokemon battle. This would surely make Serena angry.
The author can give Serena real - life fears and emotions. For example, when she is kidnapped, she can be shown to be really scared but also determined to survive. She could have normal concerns like missing her family and friends, which makes her relatable.
There's a possibility that it's a villainous trainer. This trainer might capture Gengar with the intention of using it for evil purposes, like using Gengar's powers to steal other Pokémon or cause chaos in Pokémon battles.
It could be the Marines. They've been after Luffy for a long time, so it's possible they finally manage to capture him in this fanfiction.
Maybe it's an old foe who has managed to regain power and wants to make Sora pay for past defeats. For example, a powerful dark sorcerer who was once defeated by Sora but has now found a way to capture him through some devious plot.
It could be the League of Villains. They've always been interested in taking down heroes and Deku, as a rising hero, would be a prime target for them.
No, it is not ethical at all. Rape is a serious and traumatic crime, and writing fanfiction about it in a non - condemnatory or inappropriate way is highly offensive and disrespectful to victims of sexual assault. It also promotes a very negative and harmful view of such a heinous act.