Ok hear me out, Sebastian misses Ciel even a century later and he's reliving their contract through a new master
So I watched the Kuroshitsuji live action movie (2014), and people may have disliked it a decade ago, but I think it's got potential for sebaciel angst, and I'm not seeing anyone talk about ittt
The movie is set in the 2020s, 130 years after the events of Black Butler. Our main character is named Shiori Genpō (Shiori as in Shieru—Ciel, and Genpō meaning something like Phantom-bee, as in hive). She is a descendant of our Ciel Phantomhive and is the current Queen's Watchdog (also she presents as male in public by pretending to be her lost brother "Kiyoharu")
It was revealed in the movie that the bloodline of the Phantomhives has a "curse", which is that every Phantomhive/Genpō before Shiori created a contract with the devil, selling their soul.
This implies that ever since being under the service of Earl Ciel Phantomhive, Sebastian devoted himself to serving the Phantomhive family, even following it when they moved to the East and changed their name to Genpō. By the time the movie is taking place, he has served 6 generations of the Phantomhive clan.
Now under a new master, he was once again named Sebastian (after Shiori's pet dog. What a coincidence huh?). Sebastian must've been shocked, felt nostalgic, and once again donned the name "Sebastian Michaelis". (A tribute to his old master and old identity perhaps?)
What's particularly interesting is that Shiori is very much like Ciel, from her cold demeanor and sharp wit to her love of sweets (which Sebastian smiles secretly at).
This similarity is even more emphasized by how Sebastian dresses her, addresses her, interacts with her. (You know where I'm going with this)
In the present contract, Sebastian addresses Shiori as "bocchan", a masculine and diminutive term. Our Ciel was called bocchan because Tanaka called him this as a child. Sebastian has no reason to call Shiori as such.
Sebastian could have called Shiori by any other term: "Aruji", "Goshujin-sama", "Danna-sama", "Waka",
But no. He chose to call her "Bocchan". A name he probably never thought he would ever use again. Or perhaps it's a name he calls all of his Phantomhive masters.
Additionally, Sebastian dresses Shiori in this stupid ass bow, despite her being 17 already. He probably even delights in the fact she wears an eyepatch.
This is because Sebastian placed the mark of their covenant on her right eye. And not just that, but in fact the mark of their covenant colored her eye a bright ocean blue.
The color of Ciel's normal eye. (An homage to his young master? 🥺)
Other than this, the movie contains scenarios and dialogue that Our Ciel has had with Sebastian before (the "I taught you how to beg" scene, the "this is how humans are" and "you are the one man who must never betray me" scene).
And I like to think that whenever Sebastian pauses and stares at Shiori after she says something frightfully familiar, he is reminiscing, performing lines he's said before to revel in this lost feeling of nostalgia.
In the Kuro musical, The Most Beautiful DEATH in the World, the song Hallucination has this verse
In the song, knowing that Ciel would one day die, Sebastian wishes that he and Ciel would reincarnate "somewhere" (with the kanji 異國 meaning foreign country), where they won't feel pain. And in the movie, Sebastian gains the opportunity to serve the Phantomhives—now the Genpō— in a foreign country in the East.
Even though Shiori most likely isn't a reincarnated Ciel, as Ciel's soul has already ceased to exist, this seems to be the closest he could get to having his wish fulfilled. He doesn't want to lose his Bocchan again, and it colors his actions at the end of the movie.
I'm eating this up though, as a "reincarnation" + the family demon type scenario. This creature served Ciel's family for more than a century and now his old master is back in the form of this trans guy.
He knows they're not the same people, but his mourning and missing of Ciel seems to have seeped into his treatment and service of Shiori, as if shaping her to become more like Ciel. (It's a selfish, secret thing, but demons are known to be selfish creatures)
He is reliving his life as the Young Master's servant. But in a way he's never done with his old bocchan, he lets himself be vulnerable, lets himself be affectionate, lets himself repeat the same familiar words he's said and heard lifetimes ago, but with a tinge of something more. A softer "Yes, My Lord".
This is a second chance. He lets himself be selfish.
And perhaps in this universe, he'd do it all over again. Loving Ciel in every form, even if he's no longer of this world. He'll serve and protect Ciel's bloodline, the last remnants of his Young Master. His greatest treasure.
"My Lord, I shall be by your side. Forever."
