In these fanfics, a recurring theme is the power dynamic. With Sebastian being jealous, it can either strengthen or weaken the power he has over Ciel. If he becomes over - protective due to jealousy, Ciel might gain some power in the relationship as Sebastian's actions are more driven by emotion. Another theme is the idea of loyalty. Sebastian's jealousy could be seen as an extreme form of loyalty, not wanting anyone else to have what he considers his (in relation to Ciel). And of course, the theme of love, in a very complex and often unspoken way, is also quite common as the jealousy is often a sign of deeper feelings.
A common theme is possessiveness. Since Ciel and Sebastian have a rather special bond, any threat to that bond can trigger possessiveness which is often expressed as jealousy. Another theme could be the fear of loss. Ciel, having lost so much already, might be terrified of losing Sebastian to someone else, and this fear can manifest as jealousy.
One possible aspect of 'ciel x sebastian jealous fanfic' could be the exploration of complex emotions. In many such fanfics, the jealousy might stem from the unique relationship between Ciel and Sebastian. For example, if there's a new character introduced, Ciel might feel jealous of the attention Sebastian gives to that character. It can add an interesting layer to their relationship which is already full of mystery and power dynamics.
One aspect could be the complex relationship between Sebastian and Ciel in the fanfic. Sebastian's jealousy might stem from his possessive nature over Ciel. He is Ciel's butler but also has a deeper, perhaps unspoken bond. This jealousy could add a new layer of tension to their interactions, for example, when other characters get too close to Ciel. It might lead to some interesting plot developments like Sebastian becoming more protective or overbearing in a way that both confounds and endears him to Ciel.
One common theme is the power dynamic. Ciel, as a human, has power over Sebastian in the sense of their contract, but Sebastian's demonic powers far exceed Ciel's. Another theme is loyalty and betrayal. There's always the underlying question of whether Sebastian will truly fulfill the contract or if he might find a way to betray Ciel. Also, the theme of revenge is prominent as Ciel is seeking revenge for his family's death and Sebastian is a key part of that journey.
Another theme is the power dynamic between them. Ciel is the master, but Sebastian has his own power. Fanfics play with this dynamic, sometimes showing Ciel trying to assert more control or Sebastian subtly influencing Ciel. For example, in some fanfics, Ciel may try to break free from Sebastian's influence while still relying on him. It makes for very interesting reads.
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.
Power dynamics is also a big theme. Sebastian is a powerful demon and Ciel is a young earl. The way they interact with each other in terms of who has the upper hand, and how that changes over time is often explored. For instance, in some fanfics, Ciel tries to gain more power over Sebastian, or Sebastian uses his power to protect Ciel in unexpected ways.
A third common plot element is the power play between them. Ciel uses his position as master to his advantage to make Sebastian jealous. He might give commands that seem to favor another over Sebastian. This not only creates jealousy but also challenges the boundaries of their relationship. The fanfics often explore how far Ciel can push Sebastian before things get out of hand.
Well, a frequent theme is the exploration of their relationship. It could be the development from a simple master - servant relationship to something more profound. Redemption is also a theme sometimes, like Sebastian seeking some form of redemption through his service to Ciel. And the theme of the supernatural is always present, given Sebastian's demonic nature and the dark world they inhabit.