A powerful sorcerer who is jealous of the boy's magic and wants to stop him from using it on the females. This sorcerer might see the boy's actions as a threat to his own power or plans.
One common plot element could be the boy using magic to protect the females from some sort of dark force. For example, in a world filled with demons, the boy's magic might create a shield around the females to keep them safe.
In no way is it ethical. Stories are a form of art and should uphold good values. Using magic to manipulate females for sexual fantasies is a form of objectification and violates the basic rights of the characters. It also does not conform to the standards of good storytelling which should focus on positive character development and healthy relationships.
We can change the focus of the magic. Instead of using it for sex fantasy, the boy could use magic to help the females, like protecting them from danger or solving their problems.
A powerful wizard who is conducting some strange experiments. He thinks that by forcing a boy to be a princess, he can create some sort of magical balance in the world. He doesn't care about the boy's feelings or rights and just uses his magic to enforce this transformation.
It could be another symbiote - a corrupted one that wants to take over the boy's symbiote for its own power gain. This corrupted symbiote might be controlled by an evil organization that is trying to use symbiotic powers to rule the world.
It could be another person who also has some sort of mind - controlling abilities and wants to use the boy's powers for their own evil plans. This antagonist might try to trick the boy into using his powers in the wrong way.
The antagonist in a story is typically the character or element that stands in the way of the protagonist achieving their objective. It could be a rival, a monster, or an internal struggle within the protagonist themselves.
Often, it could be a rival lover or someone who tries to break up the main couple.
It could be another powerful wizard who wants the grimoire for himself. He might be jealous of Harry's connection to it and will stop at nothing to get it. This wizard could have a dark past and be using the grimoire as a means to gain ultimate power over the wizarding world.
In this fanfiction, the main antagonist could be a strict teacher. Sakura, being a boy with a free - spirited nature, often clashes with the teacher's rigid rules. The teacher always seems to be out to get Sakura, giving him unfair punishments and trying to suppress his creativity or individuality.