He could interact with Harry Potter, Ron Weasley and Hermione Granger. They might be curious about Ciel and his different upbringing.
In this fanfic, Ciel could potentially be sorted into a Hogwarts house, say Slytherin because of his cunning nature. Then he would have to navigate the politics within the house while also exploring the secrets of the castle. He could also get involved in some kind of mystery that threatens Hogwarts and use his unique skills to help solve it.
Ciel is surely the central figure. However, there could be other characters from the original source material that have also passed on and are in heaven. Additionally, there may be original characters created for the fanfic, such as a special angel who has a particular connection with Ciel. There might also be antagonists in the form of fallen angels or some sort of heavenly bureaucrats who oppose Ciel for some reason.
Elizabeth could also be a major character. When she finds out that Ciel has turned into a cat, she might be initially shocked but then becomes very attached to cat - Ciel. She could try to dress him up in cute cat outfits. And Grell could be involved too. Grell might mistake Ciel - cat for a normal cat and try to 'play' with him in his own rather dangerous and over - the - top way.
I'm not sure specifically as there could be many different versions of such a fanfic. But generally, it might involve Ciel's journey to heaven, perhaps his experiences, the people or entities he meets there, and how he copes with this new environment.
A possible plot could be that Claude and Ciel are both summoned to a magical realm by a powerful sorcerer. They are forced to compete in a series of challenges. At first, they see each other as rivals. But as they progress through the challenges, they realize that the sorcerer has a more sinister plan. The challenges are actually weakening the barriers between their world and a dark dimension. So, they put aside their differences and work together to defeat the sorcerer and seal the rift to the dark dimension.
Sakura could have interactions with the Weasley family. Ginny Weasley might become her friend and they could have girl - talk about magic and boys. Fred and George could also be part of her story. They might play some pranks on her at first but then become good friends. And don't forget Neville Longbottom. Sakura could help Neville gain more confidence in his magic abilities as they study together in the Hogwarts library.
In addition to Ciel and his sister, there could be new characters created specifically for the fanfic. These new characters might be friends or foes of the sister. For example, a mysterious traveler who befriends the sister and helps her in her adventures, or a villain who is after something that the sister possesses.
Obviously, Ciel is the main character. He's the one usually in the diapers, and the story often revolves around his experiences, his emotions, and how others in the Black Butler world react to his situation.
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.
One popular 'ciel loves ciel fanfic' could be 'Ciel's Self - Discovery'. It delves deep into Ciel's internal thoughts and feelings towards himself in a very unique way. Another might be 'The Unseen Bond between Ciels' which focuses on the unspoken connection between different versions of Ciel. There's also 'Ciel's Self - Love Journey' that shows his growth in self - acceptance and love.