The story can show Sebastian's vulnerability by having him physically weakened. For example, he may not be able to stand for long periods or may struggle to lift objects that he normally could with ease. His once - perfect appearance could be marred, with a pale complexion and dark circles under his eyes. His voice might also be weaker, lacking its usual commanding tone.
Sebastian's character may develop a sense of humility. Usually, he is so self - assured and in control, but when sick, he has to rely on others, which can make him more understanding of human frailty. He might also become more introspective about his role as Ciel's butler and his own demonic nature.
If Sebastian sneezes at a crucial moment, like when he's about to fight an enemy, it could make him seem vulnerable. This could change the enemy's perception of him and make the fight more interesting. For example, the enemy might think he's weak and become overconfident.
There is a story where Sebastian has to deal with a group of unruly guests at the Phantomhive manor. Despite their rudeness and attempts to cause chaos, Sebastian with his calm and efficient manner manages to outwit them and make them leave in a rather embarrassed state, all while maintaining the perfect image of a butler.
One plot idea could be Sebastian falling ill due to a curse placed on him by a rival demon. He would struggle to maintain his usual composed and efficient self while still trying to serve Ciel. Another could be that he gets a mysterious illness that makes him lose his demonic powers temporarily, and he has to rely on human means and Ciel's help to recover. Maybe he gets sick from consuming something in the human world that his demon body is not used to, like a poisoned food item that affects him in strange ways.
Some fanfictions might have a plot where Sebastian's sickness makes him more vulnerable to his demonic urges. Normally, he has great control, but being ill could weaken that. This could lead to dangerous situations in the manor. There may also be a plot where a mysterious character appears, claiming to have a cure for Sebastian, but with a price that Ciel has to pay, which then creates a moral dilemma for Ciel.
Writers often develop Sebastian's character by showing his internal struggle. Since pregnancy is not a normal state for a demon like him, he might struggle with new emotions and physical sensations. He could be confused about his role as a soon - to - be 'parent' while still trying to maintain his loyalty to Ciel.
They should also consider the context. If Sebastian is crying during a battle, it could be a sign of his deep connection to the situation or the people involved. Writers can use internal monologues to show what he's thinking while crying. For example, 'Sebastian thought to himself as the tears streamed down his face, that this was a moment he never expected to feel so deeply about.' This gives readers an insight into his emotions.
Sebastian's love, if we can call it that in his rather complex relationship with Ciel, is a very dark and twisted form of devotion. It starts from his contract with Ciel. He is bound to protect Ciel until he achieves his revenge. His 'love' is more about his loyalty to the contract at first. As the story progresses, he becomes more and more involved in Ciel's life and his emotions start to get a bit more complex. He seems to have a certain possessiveness towards Ciel, not in a romantic sense in the traditional way but more like he is the only one who can be by Ciel's side according to the contract.
Sebastian might fall for a young girl who was an outcast in society. She had a special gift of seeing demons. At first, Sebastian was only interested in her because of this unique ability. But as he spent more time with her, he started to care for her in a different way. He would secretly bring her little treats and protect her from other demons who might want to harm her. Eventually, his feelings grew into love as he realized how pure her heart was despite all the hardships she had faced.
She could be portrayed as a powerful yet conflicted character. Since she's Sebastian's daughter, she'd likely have some of his demonic traits like strength and cunning. But she might also have a softer side, perhaps influenced by her exposure to the human world. She could be shown as having a unique sense of style, a mix of the elegance of the demon world and the practicality of the human world.