In some cases, it could be a new character introduced specifically for this plotline. This new character could have a mysterious background and motives. They might be working for a larger, more shadowy organization that wants to disrupt Sakura's world. Or it could be a misdirect, and the real culprit is someone close to her who has been influenced by an outside force without realizing the full consequences of their actions.
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 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.
In a 'sakura gets poisoned' fanfiction, she might first start to feel unwell, like having dizziness and weakness. Her friends could then notice the strange symptoms and start to search for a cure.
It could be a jealous classmate. Maybe someone who is envious of Hermione's intelligence and always being at the top of the class.
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.
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 it's a new character who has a grudge against the dojo in general. They target Jack because he is one of the most prominent students. This new character could be someone who was rejected from the dojo in the past and is now seeking revenge.
Sakura would be a likely candidate. As a skilled medical ninja, she has the knowledge and ability to diagnose the poison and attempt to create an antidote.
It could be an old enemy seeking revenge. Maybe a powerful Titan who escaped Tartarus and blames Percy for his previous defeat. This Titan could have been plotting in secret, waiting for the perfect moment to strike and finally end Percy's life.
Maybe it's a new villain who is trying to make a name for themselves. This villain could see shooting Captain America as a way to gain instant notoriety in the criminal underworld. They might have studied Captain America's weaknesses and found an opportune moment to strike.
A new student in the school could be the one to save Sakura. This new student might have had similar experiences in their previous school and decides to take a stand against the bullies when they see Sakura being harassed.