Who could be the main antagonists in a 'Naruto's sister loves him' fanfiction?

The Akatsuki could be the main antagonists. Since Naruto is an important target for them, and if Kurenai is now his sister, they might see her as an obstacle in their way. They could try to use their various powers and strategies to either capture Naruto or cause harm to both him and Kurenai.

One possible plotline could be that Naruto's sister secretly helps him during his difficult times in the village. Maybe she provides him with extra training resources or helps him outwit the bullies without him knowing it's her at first.

There could be other rogue ninja groups who see Naruto's sister as a threat or a valuable asset. They might try to either eliminate her or force her to join them. These groups would be antagonists as they oppose her in her new rogue lifestyle and she has to constantly fight or outwit them.

It could be some rogue ninjas who are after Lemon for some reason. Maybe they think she has a special power that they can use for their evil plans.

It could be an original character created just for the fanfiction. Many fanfiction writers like to introduce new characters to add their own unique twists to the story.

The Foot Clan could be a main antagonist. They might see the baby sister as a way to get to the turtles, either by kidnapping her or using her to blackmail them. Another could be Shredder himself if he returns. He would surely not pass up the opportunity to use the baby sister against the turtles. And Baxter Stockman could also be an antagonist. He might be interested in using the baby sister for some of his mad - scientist experiments.

One possible antagonist could be a jealous rival among the pirate gangs. This person might envy Harry Hook's sister's skills or popularity. Additionally, a corrupt official in the Isle of the Lost who wants to keep the status quo and sees her as a troublemaker could be an antagonist. Also, if there are any ancient curses or dark spirits on the isle, they could turn against her and try to stop her from achieving her goals.

In some cases, a human antagonist could work. For example, a jealous classmate who is envious of Bella's sister's relationship with a popular boy. This could lead to a lot of petty but still impactful drama at school and in the community.

The Sound Ninjas could be great antagonists. They could use their own unique parkour - like skills to try and stop Naruto. They might be chasing him through the forest, and Naruto has to use his parkour skills to outmaneuver them.

Maybe some corrupt officials within the prison system itself. They oppose Naruto's more humane and reform - focused approach to prison management. They might try to undermine him at every turn, for example, by cutting off resources or spreading false rumors about him among the prisoners.