Maybe Draco Malfoy. He could make fun of Harry for trying to fix his hair and try to jinx his attempts, thinking it's a silly thing for Harry to focus on.
The antagonist could be Voldemort in a different form. He might possess someone close to Harry and use that person to mess with Harry's hair - fixing routine. This would be a very devious way for Voldemort to annoy and distract Harry, as Voldemort knows that even small annoyances can add up and affect Harry's focus on more important matters.
He could use a simple spell like 'Capillus Ordinatus' to make his hair neatly combed. It's a made - up spell for this purpose.
It could be a dark wizard who is jealous of the Potter family's fame and power. He sees Lemon as an easy target to get to Harry and plans to use her to lure Harry into a trap.
Hermione Granger would likely be a main character. She could use her vast knowledge to help Harry in his efforts to fix Hogwarts, whether it's researching ancient spells for repairs or coming up with new educational strategies.
Perhaps a long - lost relative of the Potter family who feels they were unjustly excluded from inheriting the manor. This relative could be jealous of Harry's status and the wealth associated with the manor. They might use dark magic to try and claim what they think is rightfully theirs, leading to a battle of magic and loyalty within the Potter family and among Harry's friends.
The Ministry of Magic could also be the main antagonist in this case. If the Ministry is corrupted or under the influence of dark forces, they could manipulate Harry's friends. They might convince his friends that Harry is a danger to the wizarding world order, and his friends, being naive or misinformed, end up betraying him.
Another possibility is a group of Death Eaters who have broken away from Voldemort's control and formed their own faction. They see Harry as a symbol of the old regime that they want to overthrow. They target Harry, but his parents, with their knowledge of the old ways and their love for Harry, are able to save him from this group's attacks.
It could be some dark wizards who want to use the centaur's parts for potions. They might be following some ancient, dark magic recipe that requires centaur hair or hooves. So they hunt down the centaur, but Harry gets in their way.
A new dark wizard or witch could be created specifically for this fanfiction. This character could be interested in using the Huntsman's skills for their own evil purposes. If the Huntsman refuses, they become enemies. This antagonist could have powers related to dark forest magic, something that the Huntsman is familiar with but in a more malevolent form.
Voldemort could still be the main antagonist. Even if Demetri has some sort of connection or change in the fanfiction, Voldemort's overarching evil plans would still be a major threat to both Harry and Demetri, especially if Demetri tries to deviate from the Death Eaters' path.
The minotaur itself could be the main antagonist. It's a powerful and dangerous creature, and if it's been set loose in Hogwarts or is guarding something that Harry needs, it would be a major obstacle for him to overcome.