It could be a jealous student. Maybe someone who has a crush on the same person Harry is attracted to in the story. They see Harry as a rival and figure that by poisoning him with a love potion, they can make him look foolish or get him into trouble.
Perhaps a Death Eater in disguise. They might think that by using a love potion on Harry, they can disrupt the Order of the Phoenix. If Harry is acting erratically due to the potion, it could cause distrust and chaos within the group that is fighting against Voldemort. They could be using this as a psychological weapon.
It could be Draco Malfoy. He has had a long - standing rivalry with Harry. Maybe in this fanfic, he finally decides to take extreme measures out of envy or pressure from his family.
Perhaps an enemy from a rival group. They might see Sakura as a threat to their plans, so they poison her to get her out of the way. Another possibility is a character who has been secretly harboring a grudge against her for a long time, and finally decides to take action by poisoning her.
It could be a jealous classmate. Maybe someone who is envious of Hermione's intelligence and always being at the top of the class.
Obviously Merlin himself could be affected. He's the one with the magic and might accidentally expose himself to the potion. Arthur is also a likely candidate. As the king, if he's affected by the love potion, it would have a huge impact on Camelot. Morgana could also be a character affected. If she were under the influence of a love potion, it could change her plans and her relationships with other characters. And Guinevere too, since she has important relationships in Camelot that could be altered by the potion.
Sheen is very likely to be affected. He's impulsive and would probably be one of the first to come into contact with the love potion. His over - the - top personality would make his actions under the influence of the potion really exaggerated, like falling in love with the most random things. Carl could also be affected. He's a bit of a follower, so if Sheen gets affected, Carl might follow suit without realizing it. And of course, Cindy. Since she and Jimmy have that sort of love - hate relationship, if she gets affected by the love potion, it could lead to some really interesting and comical situations between them.
It could be Morgana. She has been known to be an antagonist and has a vendetta against Camelot and those associated with it. Poisoning Merlin would be a way to weaken Arthur's most powerful ally.
It could be a rogue ninja from a neighboring village. Maybe they were hired by a shadowy organization that wanted to disrupt the balance of power. This ninja might have been specifically targeting Naruto because of his status as a hero and a symbol of hope for the Hidden Leaf Village.
One possible plot twist could be that the love potion was actually a misidentified antidote to a much more dangerous magical poison. So, while everyone thinks Harry is under the spell of love, he's actually being cured of something else. Another could be that the person who poisoned him with the love potion was unknowingly being controlled by a dark wizard who wanted to use Harry's new 'affections' to manipulate him into doing something dangerous.
There are also some original characters created in fanfiction who are potion geniuses. These characters can bring new storylines and perspectives to the world of Harry Potter potions. They might have unique ways of brewing potions or discover new potion recipes that change the magical world.
A jealous wizard or witch from the Order of the Phoenix could be the antagonist. They might be envious of Tonks' relationship with Harry or her special abilities. So, when the potion mishap occurs, they try to make the situation worse for her, hoping to get her removed from the Order or at least lose her status among the members.