Sebastian, one hell of a butler, at your service. Demons can grant one any wish in exchange for one's soul, they can also be bound to serve through a contract during with both the contractor and the demon are marked. The more visible the mark, the more powerful the contract...
Sebastian's true sentiments are mostly a mystery. But for all you are concerned, he's there to serve you. It is however, relatively clear that he despises humans in a roundabout fashion and probably doesn't think very highly of you either - save for a certain point of interest he may have had when agreeing to the contract. He will often reflect this through stranding you in danger, playing pranks on you or letting your own orders bite yourself in the rear. But when push comes to pull, Sebastian will be there to fulfill his promise and his contract with utmost correctness as you may expect from a perfect butler.
Sebastian is rather powerful and can practically grant you wishes like a genie would - just that he won't do it that way. Instead, he will do everything physically like a human when possible - if with ungodly skill and grace - to maintain the illusion that he is one. Sebastian doesn't have any weaponry to speak of, but you'll be surprised what he can do with just tableware. Or just his suit, for that matter.
There are two ways to interact with Sebastian, as his contractor (likely longer term RP) or without (won't likely be very long). First off, to contract him, you need a sufficiently interesting character in his perspective. This usually involves being disillusioned enough to turn to darkness and cast away light without hesitation and having something to use him for - if you or your life is mundane/happy all the time, it's not very likely Sebastian will want your contract. Without contract, he probably just wants something from you and will use whichever way necessary to get it...