There have been four butler/master pairs established so far:

1) Sebastian & Ciel
2) Grell & Red
3) Agni & Soma
4) Wolfram & Sieglinde

Each of the other three relationships serve to parallel Sebastian & Ciel in interesting ways, and when they are introduced, how Sebastian & Ciel react to the pairs, and what happens in those parallel stories may even serve to elucidate the ambiguous nature of the relationship between the main characters.

Partners in Crime: Grell & Red

The first pair introduced is Red and Grell, alias Jack the Ripper. This is the darkest of the four pairs to be sure. The duo are on a murdering spree driven by the righteous idea (on Red's part) that she is in some way dealing out justice, motivated by an obsessive personality and a trauma that she can't get over—that of losing her family. It's a clear parallel to Ciel at his darkest; in fact it becomes even more complicated when you realize that (spoiler) among others, including the man she loved, Red lost her sister, just as Ciel lost his brother.

What is established at once is that this is an unequal relationship—unequal beyond, and in a different way than, you might expect with one of them being the mistress and one the servant. In fact, their story is presented almost in the mythical way of a god becoming interested in a mortal due to some particularity of their character, or amazing talent; of course, one way or another, those stories usually don't end very well for the mortal. This is made even more explicit with the description of reapers as "death gods." At this point, with the way Grell frames his duties, he has ultimate power over the souls of those he weighs—in his words, he can decide "whether to let him live on, or to kill him" based on the assessment. Of course, this is overstating the matter on Grell's part, as in actuality, reapers are discouraged from allowing people to live.

If we translate this to Sebastian, we have a parallel to when Sebastian met Ciel—what, similarly, drew him to Ciel, the connection with death as his appearance is intrinsically tied up with Real Ciel's death. Similarly, once the contract is made, Sebastian gains the ultimate power over Ciel's soul—which might even include whether to let him live, or to eat him.

Grell becomes disappointed in Red when she refuses to kill Ciel; refuses, in effect, to let the game continue, to pursue her vengeance—so he kills her, saying as he does, "How disappointing, Madam Red. I am no longer interested... in an ordinary woman like you."

That this is one possible way the story could end for Sebastian and Ciel is in no doubt. In fact, it almost ended that way early on: when Ciel is recognized as Earl by the Queen, Sebastian asks him if he can be content with the life he's gained and abandon his vengeance. When he thinks that Ciel will abandon it, he starts to lose his "butler" form and reaches out to Ciel, obviously planning to eat him right then and there. When changes the story is that Ciel refuses to abandon his vengeance; and this moves Sebastian to an absurd degree. Still, you might argue that it is in that scene that Sebastian swears fealty to Ciel, that he in fact promises to play the game until the end, without messing around.

Even later, during the Green Witch arc, Sebastian is driven in desperation to repeating his question, when time has run out for them to remain in the village.
"Brooding over your fears and regrets, curled up in your blanket... is not what you should be doing right now. Come, get out of bed." Will Ciel abjure his vengeance, or does he still want to play the game?
Ciel says, "No."
"Oh? Well. You will abandon your position as the Queen's Watchdog then? Very well. You won't have to experience any hardship if you're no longer the Queen's Watchdog. No one will blame you for not wanting that forever. The servants will surely treat you well. However. Abandoning your revenge mid-way is against the contract. Ahhh. What an utterly boring end." Sebastian has turned from a human-looking butler into a darkness-tentacle-creature and starts to eat him. But even then, he doesn't stop trying to remind Ciel of his revenge, in a last-ditch effort to not end the game just yet. Sebastian asks Ciel why he actually made a contract with him, and Ciel reminds himself (or perhaps realizes for the first time?) that the reason he did it was, whatever he said or thought at the time, actually for himself. He breaks free of the cage and runs toward Sebastian.

Ciel then says to Sebastian, "you... really tried to eat me just now, didn't you."
Sebastian says, "No, no, I was only 90% serious." (Real reassuring, Sebastian.) He then reminds Ciel that he is both a demon and a butler.

Grell compares the relationship between him and Sebastian to that of Romeo and Juliet, and that along with his comment that he is an actress, casts everything about his relationship to Red as an extended act—this is made even more clear in the anime, where Grell is introduced as a butler in an earlier episode, and plays it up to a theatrical extent. Similarly, one could see Sebastian, and his butler aesthetic, as a similar type of act, where the "play" being put on is more important than the people involved; it is all about the artifice. Grell actually replays the famous balcony scene, saying to Sebastian "you and me shall be the cast in this show," and continuing to riff off of it: "why don't you throw away the unlucky name your master gave you, and just focus your attention on me!" —wherefore (why) art thou Sebastian?

Sebastian gives an answer: "all it takes is that one sentence. From the time when my master addressed me as 'Sebastian' that sentence became the newly formed contract. Since that day, I became 'Sebastian.' I made a vow to the moon."

In Romeo and Juliet, Juliet says, "O Romeo, Romeo! Wherefore art thou Romeo? Deny thy father and refuse thy name..." continuing on to entreat Romeo to abandon the fact that he is a Montague and can therefore only ever be her enemy.

Romeo answers "call me but love, and I'll be new baptized; henceforth I never will be Romeo."

The way that Sebastian plays off of this exchange is clearer in the anime, and probably in the official translation too. Grell sets up the unstable dichotomy of death god/demon, who must always be natural enemies, but then positions Ciel, and Sebastian's connection to him, as what will keep them apart, and what Sebastian will have to refuse to get together with Grell.

Sebastian's answer actually uses Romeo's words, saying that in meeting Ciel and in the creation of the contract, he was newly baptized as Sebastian; in other words, Grell can't be Juliet because Ciel is.

In Romeo and Juliet, Romeo begins by swearing his love on the moon, and Juliet interrupts him, saying:
"O swear not by the moon, the inconstant moon,
That monthly changes in her circled orb,
Lest thy love prove likewise variable."

So in saying that he's swearing by the moon, Sebastian is actually playing along with Grell's reenactment of Romeo and Juliet, while still refusing Grell's casting of himself as Juliet, and putting Ciel in that place instead.

Soulmates: Agni & Soma

Agni is set up from the beginning as an incredibly good butler, just as Grell was (to Sebastian's disapproval) an incredibly bad one. The usual, 'oh no the whole house has been messed up' that was set up from the very first arc is flipped around, with Agni having done all this work incredibly well... this gets on Sebastian's good side immediately. Agni is also able to get the servants to work together and not break everything, which completely baffles Sebastian, leading to one of his awesome shocked faces. Agni explains his personal philosophy to Sebastian, and then explains that before he met Prince Soma, he was a very different kind of person. "Before I met the prince, I was an idiot that can be said to be hopeless. What I owe him can never be repaid in this lifetime." He explains how he had no faith, due to seeing his father misusing his position, and how he then continued to do bad things, saying "I took advantage of my social status and committed sins every day." But just as he was about to be executed for his crimes, Prince Soma arrived to say him, saying, "I've heard you are a wild person that is as strong as an untamed beast," and choosing to take responsibility for Agni. As he cuts Agni's hair, Soma says, "the 'you' who existed before today is now dead. You're hereby reborn with a new name and life." Agni interprets this memory, saying that from that day Soma became his god. He says "that day, I definitely saw a god emitting holy light inside of the prince's body! I swerved the prince ever since that day... the prince is my king and my god!" He continues to say that he will do anything for Soma, even if it costs him his life.

While Agni is set up as literally the nicest person in Black Butler, his own story places him as a reformed sinner who has found god in the man he serves. Still, even in his "reformed" state, his dedication to Soma is shown to be potentially dangerous, as his fervor leads him to do criminal acts in Soma's name. There is no unequivocal good here, it must be pointed out. Still, everything that Agni affects, day to day, seems to lead to having friends and happiness because he's just such a nice person. This seems like a pretty weird comparison to Sebastian, but he certainly is a parallel to Sebastian. Not only do they both take pride in cooking, and are incredibly good at it, not only do they both have superhuman strength, but the story of Agni meeting Soma has some interesting resonances with Sebastian's own.

Going back to Sebastian's description of his meeting Ciel to Grell, earlier on, a flippant reply that takes from Romeo and Juliet and the wild overstatements made by those in love can also be interpreted through the lens of someone who has found a god. The devotion becomes religious. There is, of course, the baptism mentioned in Romeo and Juliet, and by Sebastian in relation to his own story; Agni, as well, tells the story of a baptism, in which he gained a new name and a new life.

Soma's words when he first met Agni are also interesting. "I've heard you are a wild person that is as strong as an untamed beast." The connection to Agni's former existence, as an "untamed beast" is explicitly similar to Sebastian, who first appears to Ciel as a number of beasts, and is referred to as a beast multiple times, including in the Green Witch arc, when Ciel tells Sebastian that, in destroying the evidence of the laboratory, he has his permission to "feel free to go crazy like a beast." (Sebastian's reaction is notable. He thinks to himself, "My master. You're misunderstanding something. What I find most entertaining is playing the butler in my game with you. It's not behaving like a frenzied beast. However, if that's how you like me... I'll act accordingly.")

Further, this sounds really similar to the meeting between Gilgamesh and Enkidu inGilgamesh. Gilgamesh is described as "two thirds god, one third man" and like Soma, he starts the story thinking a lot of himself; to say that he's arrogant is an understatement. Enkidu is created as someone who can temper this, basically Gilgamesh's soulmate, and spends the first part of his life as a literally "untamed beast" of a man, living with the other wild animals and having more in common with them than with humanity. But after he's shown human things through having sex with a priestess who comes to tame him, he can no longer go back to that state of innocence, wildness, or beastliness... now he is unavoidably human, and that can't change. He and Gilgamesh have a number of incredible adventures, and Gilgamesh thinks that together they can do anything, but Enkidu eventually dies, and the rest of the story deals with Gilgamesh's grief and anger at what he feels is the unfairness of death. He goes to extremes, and journeys for a long time, to try to become immortal himself, but discovers that like anyone who is mortal (even one-third mortal) death will inevitably come for him. Still, he eventually gains some measure of peace and grows as a character. Perhaps Soma is also on a similar journey. And, if Soma is a parallel for Ciel, who has been going on his own angry journey, through the manga, against the fact of death, both of his parents and of his other half—his twin brother—perhaps Ciel himself might come to some measure of peace as well.

So, Agni is the epitome of a butler, and as he dies, he thinks, "Master Soma, that day, when you saved me, you were probably just acting on a whim. But from that day forward, you have always been the sun in my sky."

(In Romeo and Juliet, when Juliet appears, Romeo says,
"But, soft! What light through yonder window breaks?
It is the east, and Juliet is the sun.
Arise, fair sun, and kill the envious moon...")

Agni continues, "no matter the circumstance, I remained bound to you. As your orders were the light illuminating my path. What's more, just by exuding your shining confidence down from the sky, you have saved more lives than you will ever know... because you were my sun, were there for me, my days could finally begin. ... I was... your khansama was... extremely happy."

Soma is, in Ciel's words, who he might have become if not for "that day". Soma's description of his life in the palace, where he as the youngest child had everything he could ever want, materially, but felt as though he was cut off from expressions of love, is what motivates him to cling so tightly to Meena, who was always there for him, though she was secretly resentful. (Oh my! Perhaps the way that Madam Red was always there for Ciel, and yet secretly resented his existence because she was jealous?) While Soma originally saved/hired Agni just because he was supposedly really good at making curry, the butler has come to mean everything to him. (This reminds me of Ciel's meeting with Klaus, where he says that one of the reasons he keeps Sebastian around is because he makes the best sweets ever. He's being flippant, it's not a serious reason by any means—and Sebastian couldn't cook when they met—but it's still interesting.) Soma says to Agni, later, "Ciel reminds me of my younger self. I would always be chasing after the love of my parents and Mina, but I never paid attention to the affection you were giving me the whole time." He says, "No, not quite the same. Even though he is aware of al the affection he is receiving, he refuses to accept any of it. It's as if he is afraid of obtaining happiness. Ciel says that he lives for the sake of revenge. That's why he lives a life of solitude." (Of course, that's the only reason Ciel is still alive instead of being eaten by Sebastian for having broken the contract.)

In this iteration of butler and master, the butler dies for the master—something that Sebastian states that he finds admirable. Indeed, this has also almost happened in Sebastian and Ciel's own story. When they're on the Campania, Sebastian gets stabbed by Undertaker when he's trying to protect Ciel, and could very well have died in the process. He didn't, and so the story continues...

Parent and Child: Wolfram & Sieglinde

Sieglinde is a genius brought up believing one story—that she's the Green Witch, someone with a special duty handed down through the generations to keep her village safe—when really she's being manipulated by an old woman (her mother, though she doesn't know it) into making a gas that will cause death and destruction. This is rather interesting. If Sieglinde's story is a parallel to Ciel, just as Madam Red and Soma's stories are in a sense a parallel with Ciel, then Ciel might be believing a false story too. He, like Sieglinde, takes pride in his position—in his case, of being the Guard Dog, and keeping order in England. It's couched as a special duty, but perhaps he, too, is being manipulated by an old woman (the Queen? Hm). It's certainly made clear that the Queen has her own aims, and that she's interested in preparing for war. The idea that she might even be the one behind his parents' murders isn't so far off, and indeed, in the anime, she *was*. On an even more disturbing note, perhaps Vincent Phantomhive was more manipulative than he appeared...?

Wolfram is in a similar situation as Finny. Both were used in experimentation in Germany, and both didn't have a name until they met Ciel, in Finny's case, and Sieglinde, in Wolf's case. ("For this mission, we gave you a name," he was told. "Lieutenant Wolfram Geltzer.") That's a pretty long list of people who were given new names, including every single butler—besides Grell—to date. (But, quite interesting is the recent reveal that reapers are, along with their names, which most of them use, also referred to by a number code within the organization.) (As a side-note, Wolfram is responsible for Sieglinde's crippled feet, just as Sebastian is responsible for Ciel's blind eye, but beyond that obvious similarity, I'm not sure it has any significance.) Wolfram describes his life before Sieglinde as being colorless, and that they had a duty, and acted, but without ever knowing the reason. This "patriotism" is then set against the personal loyalty he ends up having toward Sieglinde.

Wolfram's past, on learning how to be a butler, is similar to Sebastian's, as he is baffled by having to deal with a child and with the idea of having to play with her and deal with her whims, instead of just killing people. He thinks, "after meeting you, for the first time... I discovered the bright colors of the world... and felt the desire to protect something. I found a reason to fight." He thinks this as he has been wounded, and reaches for Sieglinde through the haze of his memories. There is even a shot of their hands reaching out toward each other, as Sebastian and Ciel reached out to each other when Sebastian was wounded on the Campania, and he slipped into his early memories of their time together.

Wolfram is saved though, and Sebastian says, gently, "Mr. Wolfram. A butler... isn't allowed to die before their master." (Agni should've listened to you!/Tanaka. But Agni, of course, is really the only one who can get away with disagreeing with Sebastian, and still change his mind.)

This story isn't really told to it's end, but it's different from the others in that Wolfram was originally working at cross-purposes to Sieglinde, and was in effect on an undercover mission to use her. But instead, he ended up throwing in his lot with her, and abandoning his army. Furthermore, at least to the present moment, though he would give his life for Sieglinde, he doesn't have to. Sieglinde actually forgives him for his deception, realizing that his actions since then show his care and his regret.

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