Sebastian takes on the appearance of a tall, handsome male adult with black hair and red eyes. He typically dresses in an outfit that consists of black trousers, a tailcoat, and a vest. He has the Phantomhive crest on his shirt cuffs and on his tie. He also sports a pocket watch with a chain, and a chained silver lapel pin bearing the Phantomhive Crest, traditionally worn by the house's head butler.
His white gloves conceal the mark of his Faustian contract with Ciel Phantomhive on his left hand. He occasionally removes these, revealing black fingernails.
Sebastian's demonic form has never been fully shown. Details of this form include high-heeled stiletto boots, a shower of black feathers, sharp claws, glowing fuchsia irises, slitted pupils, and longer, sharper teeth. Prior to transforming, Sebastian says several times that his form is unpleasant to look at. Just looking at it drove Ash Landers to hysterics, screaming that he was "filthy."
Sebastian often contemplates about the behaviors and attitudes of human beings. He finds humans, in general, to be "difficult creatures," but he adds that their struggles and lofty goals make them interesting. He is intrigued about how insatiable they are, as they are willing to "drag others down to get what they want, even in death."