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.
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.
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.
Maybe they get separated at a young age due to some family drama. Years later, they reunite and have to learn to be brothers again while also dealing with the changes in their lives. For example, one has become a successful artist while the other is a detective, and they combine their worlds in unexpected ways.
Give them distinct personalities. For example, one can be more outgoing and adventurous while the other is more reserved and intellectual. This contrast will create interesting dynamics between them.
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.
The twin brother's character might be developed based on his past. If he had a difficult childhood, it could make him more reserved or cautious. But as he gets to know Naruto and Sakura, he starts to open up. He could start as a loner type, but as he gets involved in their adventures, he becomes more social and confident. He might also develop his character by exploring his own unique abilities and how they can be used for good.
One popular 'ciel twin fanfic' is 'The Lost Twin's Return'. It delves into the idea of what would happen if Ciel's twin suddenly reappeared. Another great one could be 'Twin Bond'. It focuses on the unique connection between the twins.
Another way their personalities could differ is in their sense of humor. Ruby has a more light - hearted and sometimes goofy sense of humor. The twin brother could have a drier, more sarcastic sense of humor. For instance, when Ruby makes a joke, he might respond with a deadpan comment that shows his different comedic style.