It could be another powerful demon who sees the twin brother as a threat. Since he's new to the demon world, this more experienced demon might want to eliminate him.
Maybe it's a human who has knowledge of the supernatural and has sworn to destroy all demons. This person could target the twin brother specifically because of his connection to Ciel. They might see Ciel as a way to get to the twin brother, and vice versa. The fanfic could explore the moral gray area of this antagonist's actions, as they are trying to protect the human world but also causing harm to the twin brother.
Well, perhaps the story starts with the discovery of his demonic nature. As a demon, he could be drawn into the dark underworld of demon politics. He might be forced to make deals with other demons to protect Ciel. His journey could also be about finding a way to reverse his transformation, all while dealing with the fact that he's different from his human twin now.
Maybe the main antagonist is a group of demon hunters. They sense Ciel's transformation into a demon and see it as their duty to eliminate him before he becomes too powerful. They hunt him down relentlessly, making Ciel's life as a new demon very difficult.
Obviously, Ciel and his twin are the main characters. Often, Sebastian, Ciel's butler, also plays an important role as he is involved in Ciel's life and would be affected by the presence of the twin.
It could be an ancient monster that has a grudge against the family of Poseidon. Since Percy and his twin are children of Poseidon, this monster might target them specifically. Maybe it's a sea serpent that was once defeated by Poseidon and now wants revenge by going after his descendants.
Orochimaru would be a great main antagonist. He's always been interested in power and the secrets of the body. Naruto's half - demon fox nature would be a prime target for his experiments. He could try to extract the power of the demon fox from Naruto, or use Naruto as a way to create his own brand of super - soldiers with demon - like powers.
Ciel's twin brother in Black Butler fanfiction is a really interesting concept. It can bring a whole new set of possibilities to the story. He could have a different personality, like being more outgoing while Ciel is more reserved. This would lead to different interactions with the other characters.
In many Black Butler fanfictions featuring Ciel's twin brother, the story often takes a different turn. The twin can be used to add new layers of drama. For example, he might be the 'good' counterpart to Ciel's more jaded self, or he could be a rival in the power struggle within the Phantomhive family. Some fanfictions explore how the twin's presence would have changed Ciel's experiences, like his time at the manor or his dealings with Sebastian. It gives fans a chance to rewrite the story in a more complex and interesting way.
In some fanfics, the twin brother might have a special bond with a female character. For instance, Sakura could be intrigued by the twin brother's different personality compared to Naruto. She might initially be confused about her feelings towards both brothers, which could add a romantic sub - plot to the story along with all the ninja action.
An ancient dark magic entity that has a vendetta against the Potter bloodline. It targets Harry and James, and they have to uncover the secrets of their family's past to defeat it. This antagonist could bring in a lot of mystery and exploration of the Potter family history.
Voldemort could be the main antagonist. He might have seen the potential in Harry's twin and decided to get rid of the twin to weaken Harry or use the twin for his own dark purposes.