Sebastian is a demon with mysterious originals. It’s really hard to say anything about the past of Sebastian because he doesn’t reveal anything about his past. And since the knowledge of his past is not important for the story, I feel we fans won’t get to learn that much about him any soon. But there are some interesting things we can say about him based on the evidences revealed in the manga.
So Take A Look At The Interesting Theory About Sebastian
First of all, a fact that has been revealed is that Sebastian was never human. Yana Toboso once said in a blog post that he’s a demon by nature. And he has a lack of understanding when it comes to human emotions and he often expresses surprise at the human actions, it fascinates him. So it’s pretty safe to say that Sebastian doesn’t have much experience with humans. Although this demon has been around for a long time already (when compared to the lifespan of a human) since he considers the human life span as a very short amount of time like a blink of an eye.
Also what’s more interesting is that Sebastian didn’t have many long-time contracts before, or if any at all. It also doesn’t appear to be necessary for a demon to form a contract with a human in order to consume their soul (William T. Spears once implied something similar and said that Sebastian tried to consume Ciel’s soul twice since forming the contract with the boy without fulfilling the contract).
So It’s safe to say that Sebastian used to eat any soul that he would come across to satisfy his hunger and without bothering about forming a contract with the human for the most part of his demon life.
But then he probably got bored with this approach of consuming a human soul with such ease because it didn’t present any challenge to him. So he changed his approach and started choosing which soul he would eat. He would even go to the extent of forming a contract with the human because it would present him a challenge and also save him from the trouble with the Shinigami. Forming a contract provides him an opportunity to manipulate the human and thereby making the soul of a human more delicious.
A contract makes things more challenging because Sebastian now can’t just eat any soul, he needs to wait for the right time and be patient. He actually has to earn the soul and his hunger makes soul tastier in the end. The contract thing is more of a game to him just like a lion plays with its prey, a demon plays with a human. A contract has terms which are rules and he has to play by the rules in order to make the game more fun.
This new approach of following the rules and then taking humans’ souls is new to Sebastian. It is entirely possible that Sebastian may have had other masters before Ciel Phantomhive, but I think the contract he made with Ciel is still something special. His contract is probably more challenging and this makes it more enjoyable than any other contracts that he may have had before in the past.
Also, based on the facts it’s safe to say that Sebastian didn’t have a connection to any of the Phantomhives before. It’s because he seems too clueless about them. Having no knowledge about any of the prior Phantomhives. All he knows about them comes from his contract with Ciel. He just appeared by chance on that fateful day.