It could be Draco Malfoy. He might not like the idea of Harry getting a second chance at the OWLS and tries to sabotage him. Maybe he spreads rumors about Harry not being worthy of a retake.
The Ministry of Magic could be the antagonists. They might have strict rules against retaking OWLS and try to stop Harry. They could be worried that if Harry is allowed to retake them, other students will demand the same. So they send officials to make things difficult for Harry during his retake.
Well, maybe Harry discovers he didn't do as well as he hoped on his OWLS the first time. So he decides to retake them. He might study new spells and magical theories that he missed before. He could also face some challenges from other students who don't like the idea of him getting a second chance.
There could be some old - fashioned professors at Hogwarts who are against Harry's unorthodox ways of making his mark. They believe in the old - established ways of teaching and practicing magic and see Harry as a troublemaker who is disrupting the order of things at the school.
It could be a group of dark wizards who are jealous of Harry's status and see Lemon as a way to get to him. They might try to kidnap her or manipulate her to get Harry to do their bidding.
The Thalmor could be great antagonists. Their strict control over magic and their anti - human beliefs could put them at odds with Harry. They might see his different form of magic as a threat to their power in Skyrim.
One possible antagonist could be a corrupt wizard in a high - ranking position in the Ministry of Magic's financial department. This wizard might be trying to cover up his misdeeds and would see Harry as a threat to his scheme.
Voldemort's remaining followers could be antagonists. They might target Harry's little sister thinking they can use her to get to Harry. They could try to kidnap her or use dark magic on her.
Scarecrow could also be a main antagonist. His fear - inducing toxins could be a real problem for Harry. Harry would have to use his magic to counter the effects of the fear toxin and also figure out a way to stop Scarecrow from spreading it all over Gotham.
In fanfiction, it might symbolize Harry Potter's departure from a familiar situation. Owls in the Harry Potter universe are important for communication and connection. By taking his owls and leaving, it could represent him cutting ties with the wizarding world he has known, perhaps to start a new life away from the chaos or expectations. It could also be a sign of his independence, as he is taking something that has been part of his life in Hogwarts (the owls) and forging his own path.
Another possibility is that there could be a new character introduced in the fanfiction, a mastermind who manipulates Harry into believing that his family is a threat. This new character could be the main antagonist.
It could be the other Titans who don't want a half - wizard, half - Titan like Harry to exist. They might see him as a threat to their pure Titan bloodlines or their power structure.