Ok, so I will be referring to the boys as the following: Lord Ciel (LC), and
Young Master (YM, our Ciel).
So let me start from the beginning with all of this until when the fire was happening. After discovering his parents' dead body, YM rushes to find help. He finds Tanaka, and Tanaka seems quite shocked. So in the scene where Tanaka is telling YM to escape, the translation from Japanese to English is "Please escape, Ciel sir it's too risky for you." However, in the Japanese, it reads "Please escape young master! Something has happened to lord Ciel."
So there's that, after YM and LC are taken an auctioned off. Whilst paying for YM and LC, the man says "This is worth more than two people" This could suggest either YM is rare because he's a noble, or both YM and LC are noble so they're worth more than two people together!
If we go ahead to the next visual, we had the prison which would be when they were in Germany and YM was cursed and poisoned. If I remember correctly, he imagines a scene where he is in the cage, holding his brother's (LC) hand, speaking in past tense saying "I still had you." Eventually, we see LC being dragged to the altar for sacrifice, as YM calls out "Ciel" proving the boy who was killed is the real Ciel Phantomhive. YM is still in the cage, watching distressed! After witnessing his brother's death, YM knows he has nothing left. This completes the two terms for summoning a demon; a person willing to give up their soul for a wish (YM) and the price to cross the river (LC), thus Sebastian appears.
The only reason this was successful, was because the cultists weren't prepared to sacrifice their own souls; only the souls of children. As Sebastian states, if you cannot sacrifice your soul, you cannot summon a demon. Sebastian takes LC's soul that was given to him so he could appear. This is why Sebastian gave YM the choice of the contract because Sebastian had already been paid. As expected, YM sacrifices his own soul for power. After all the killing of the children (classic Kuroshitsuji scene), Sebastian asks YM what his name is. YM pauses for a second looking at LC's dead body, with which he replies; Ciel Phantomhive.
The next one isn't easy to explain, but I'm going to try because it really proves my point.
So remember this boy that was addressed as Ciel. Notice the bruise underneath his eye. If you look through the flashbacks, you will notice this is a constant detail for the altar boy, but the boy who made the contract does not have a bruise under his eye and never did have. That is the only reason we know that YM is not Ciel. Once YM tells Sebastian he is Ciel, Sebastian gives a slight laugh while saying "very well", therefore proving he knew YM was lying. Time skip to a bit later, Sebastian calls YM out saying, "You told me not to lie, but you seem to be quite the liar yourself." YM glares at Sebastian saying "Shut up, you have no right to bring that up," so the flashback ends here
So in conclusion,
- LC is killed, used as the price to summon a demon
- With YM also willing to sacrifice his own soul, a demon appears.
- YM is offered a choice because the demon was already paid.
But that leaves one issue, why would YM take the name "Ciel Phantomhive"? Well, it's important to note, YM was the youngest twin, not the heir to the Phantomhive name, LC was. YM was also sick as a child whilst LC was outgoing and healthy, but that alone isn't the reason. LC seemed to get more attention than YM, Lizzy even states "Madam Red loved Ciel the most out of all of us" (the online version isn't translated correctly), which is a major blow to anyone's confidence, so the ideas of why YM took the name "Ciel" are:
- YM was the son who would never really get anything
- Nobody seems to like him as much as his outgoing healthy brother. Madam Red, Lizzy, Baron Kelvin
- So perhaps he takes his brother's name so people will not see him as weak, and respect him more.
- Or the guilt he feels that his brother deserved to live, not him. YM feels it would have been better if LC came back instead of him.
There are too many reasons to nail one down right now, we don't have enough information, but YM does seem to have survivor's guilt:
Frustratingly, YM is interrupted before we know who he is speaking of, but it's most likely LC. In chapter 25, in YM's dream, he states he doesn't want the dead (LC) to forgive him. It shows he believes LC's death is on his hands.
That's all I have right now.