Due to lack of canon information on Sebastian, this interpretation relies heavily on Christian theology, Western history and my own headcanons. This describes how I write him by default and with the revelation of canon content, it will be subject to change.
A self-important ‘anthropologist’ in the guise of a butler, Sebastian stepped down into the human world to form a contract with the Earl of Phantomhive. An impeccable servant though he is, the demon cannot be separated from his heartless and sadistic nature. With his obedience to the Earl as nothing but a mere passing of time, Sebastian only seeks out the greatest pleasure that he can until his services are finally compensated.
Once he had tasted the joys of heaven, only to be neglected by the Lord who had once promised his eternal love to all his creations. This, coupled with his developed free will, drove him and other angels to rebel against his power – this of course ended up in eventual defeat and they were cast down to the realm of mortals.
Masking himself as a human, Sebastian grew tired of the persona of his grand personas. From advisors to pharaohs or tutors to royal households, and once a courtigiana, messily sampling human souls became tiresome. Only since the 4th century had Sebastian finally adopted the custom of contracts that other demons had claimed were “all the rage that millennium”.
Sebastian’s first contract was the only that he had ever seen as worthwhile. Promising his master a life of wealth and glory in poetry, it was all over too soon and Amulius Tatius became a forgotten name. It was this greed driven human that piqued Sebastian’s inspiration – this was the man that sparked his desire to explore humanity.
With many years to himself, Sebastian found interest in metaphysics and philosophy. That is, messing with others who held those interests. As a near omniscient being, Sebastian takes pleasure in leading those who ponder on the secrets of life down dead-ends, his oration skills making him appear entirely trustworthy. However, the matters of this universe don’t exactly interest him at all.
Perhaps something he picked up from his first contractor, Sebastian is far more well-read than any demon he’s bothered to keep in touch with. He can absolutely bore one of his ‘peers’ with an in depth analysis on Don Quixote, despite none of them caring about chivalry, or completely destroy Milton’s interpretation of their own plights. Most nights, when his work is solved and the household is asleep, he sits at his desk and reads. Not once has he ever left a book in his room, returning the pieces to his master’s study every night.
