Another theme is the struggle between Ciel's human side and his demonic nature. Fanfics may show him grappling with his emotions, memories, and moral compass in the face of his transformation. For example, he might question his past actions and how they fit into his new identity. It makes for a very interesting character study.
A lot of fanfics also focus on the relationship between Ciel and other demons in the 'Black Butler' universe. This could involve power struggles, alliances, or even romantic relationships. Some fanfics might create new demon characters that interact with Ciel in unique ways, adding new dimensions to the story.
One common theme is the exploration of Ciel's relationship with Sebastian as a demon. How their contract changes and how Ciel adapts to his new demonic nature. Another theme is Ciel's struggle for power in the underworld, as he is now a demon. It often involves him facing other powerful demons.
One common theme is the power imbalance. Sebastian is a powerful demon, and Ciel is a young, mortal master. This creates a lot of tension and interesting situations where Ciel is constantly trying to hold his own against Sebastian. Another theme is loyalty and its limits. Sebastian is bound by a contract, but his true motives are often questioned.
The theme of sacrifice also shows up a lot. Ciel may sacrifice his own humanity in order to achieve his goals, and Sebastian may sacrifice his own freedom in a way by being bound to Ciel. And there's always the theme of the supernatural, with Sebastian being a demon and all the dark magic and otherworldly elements that come with that in the fanfics.
One popular theme is the complex relationship between Ciel and Sebastian. It often explores the power dynamics, with Sebastian being a demon bound to serve Ciel. Another theme is the dark and mysterious world they inhabit. Many fanfictions delve into the supernatural elements of the Black Butler universe, like the existence of other demons and their motives. Also, the idea of Ciel's revenge and how Sebastian aids him in that quest is a common theme. It shows how their goals are intertwined despite their very different natures.
One common theme is Ciel's revenge. Since his backstory in the original series is about seeking revenge, many fanfictions explore this further. Another is his relationship with Sebastian, so there are a lot of fanfictions that focus on their master - servant dynamic in different situations.
One common theme is the vulnerability of Ciel. When he's sick, his usually strong and stoic facade crumbles, and we see a more human side of him. Another theme is the loyalty of his servants. They go to great lengths to care for him, highlighting the bond they have. For example, Sebastian might use his demonic powers in a different way to heal Ciel rather than just for his usual tasks.
One common theme is identity. Since Ciel already has a complex identity, adding a twin doubles that complexity. Another theme is family, exploring the relationship between the twins and how it affects their actions.
One common theme is the exploration of gender roles. Since Ciel is female in these fanfictions, it often shows how she defies or conforms to the Victorian era's expectations of women while dealing with the underworld and supernatural elements. For example, she might have to balance her ladylike image with the need to be a strong and decisive leader like the male Ciel was.
Family or surrogate family is also a theme. Given Ciel's tragic family situation in the original, fanfictions may introduce new characters or develop existing relationships in a way that gives baby Ciel a sense of family. For instance, some stories might have the servants of the manor taking on more parental roles towards the baby Ciel, creating a warm and loving environment for him to grow in.
One common theme is mystery. Ciel's eye is already quite mysterious in the original work, and fanfictions often delve deeper into it. For example, they might explore the origin of its power in more detail.