A power - hungry fairy could be the main antagonist. This fairy might see Artemis as an upstart and a threat to their own position in the fairy hierarchy. They could try to undermine Artemis at every turn, setting up traps and spreading false rumors about him in the fairy world.
There could be a dark force, like an ancient evil that has been awakened by Artemis' transformation into a fairy. This dark force might target him specifically because he is a unique being, part human and part fairy, and it wants to use his power for its own evil purposes.
Perhaps Artemis, now a fairy, could use his knowledge of the human world to bridge the gap between humans and fairies in a more positive way. He could introduce human technology to the fairies, while also learning and sharing fairy magic with humans. This would create a unique story where he is sort of a cultural ambassador between the two worlds.
Perhaps a jealous classmate who discovers Artemis's connection to the fairy world. This classmate wants to use that knowledge to gain power and popularity, so they start causing trouble for Artemis and his friends, trying to expose them to the wrong people or get in the way of their fairy - related investigations.
There might be a group of anti - magic fanatics as the main antagonists. They view Natsu's new and extremely powerful magic as a danger to the natural order of the magical world. They would try to eliminate Natsu and any other powerful mages in Fairy Tail who support him, believing that magic should be limited and controlled, not allowed to reach such overpowered levels as Natsu has.
Maybe the main antagonist is an ancient evil that has awakened. It has the power to manipulate Artemis's mind, making her kidnap Percy against her will. This evil entity wants to use Percy as a sacrifice to gain more power. So, while Artemis is the one who physically kidnaps Percy, the real force behind it, the ancient evil, is the true antagonist.
Maybe a powerful warlord from Negan's world who somehow follows him into Artemis' realm. This warlord, with his own thirst for power and destruction, would oppose both Negan (if he starts to side with Artemis) and Artemis herself, wanting to take over the new world he has found.
Perhaps a powerful and corrupted demigod could be the main antagonist. This demigod might be jealous of Artemis' power and popularity among the other demigods. He devises a plan to harm her, and Percy, being loyal to Artemis, has to give his life to save her from this rogue demigod.
Artemis Fowl is the central character, a brilliant and somewhat conniving young boy. Another important character is Holly Short, an elf in the fairy world. She is a strong and capable female character. There is also Butler, Artemis's loyal bodyguard. He is highly skilled in combat and protecting Artemis. These characters play crucial roles in the story, with their interactions driving the plot forward.
A powerful sorcerer who wants to control the power of the fairy of darkness for his own evil purposes could be a great antagonist. He might try to trap her and force her to do his bidding.
It could be an evil fairy king. He might see the poison fairy as a threat to his rule. Maybe he believes that her powers are too dangerous and wants to either control her or get rid of her. He could send his minions after her and try to spread false rumors about her in the fairy community.
The main antagonist could be a powerful and ancient fairy who has been banished long ago but now returns to seek revenge on the fairy world. And Lucy, being a new and important part of the fairy world, becomes a target. This antagonist might have unique and dark magic that makes it extremely difficult for Lucy to defeat. Also, the antagonist could have some sort of grudge against the fairy queen or king, and Lucy is caught in the middle of this power struggle.