"I'm one hell of a butler.''
Sebastian's one purpose is to be the butler to his master, Ciel Phantomhive- and of course, a stubborn and weak master like his needs certain characteristics in his butler to balance out the inequalities. In comparison to Ciel, Sebastian is calm and collected, and often coolly indifferent in his feelings and actions- indeed, he faces dangers for Ciel without so much as batting an eye. He looks at the world in a mildly interested, detached way that always comes off as polite to others. His intellect, manners, and abilities are second to none, so much so that he is adept at anything and everything he knows and attempts. To anyone who sees him in action, he could possibly be considered the perfect butler.
But everyone in the Phantomhive household knows that he's not quite perfect. Sebastian's dry wit and sarcasm often spring up when he's particularly displeased, which is pretty often, owing to the messes the rest of the Phantomhive staff get themselves into (that he always has to clean up). He gets easily exasperated, especially when his work is interrupted.