Sebastian is a dedicated butler to Ciel Phantomhive and upholds a strong sense of duty to his master and household. He faithfully follows and carries out every one of Ciel's orders, while ensuring that the Phantomhive household runs smoothly. He does not sleep—for demons deem it as a form of luxury—and thus, he frequently performs tasks for Ciel at night.  One of his quirks is that he will always respond to Ciel's demands with the saying "Yes, my lord."

Sebastian predominantly appears as a courteous and impeccable servant. However, he is, in actuality, callous, heartless, and sadistic; he is ruthless when attacking others on Ciel's orders, and constantly puts Ciel in nonfatal danger for his own amusement. Sebastian usually expresses his diabolic nature through his dark, sarcastic humor and inability to see good in anyone. He is known to not take instances in which Ciel is in danger seriously, such as kidnappings. However, he displays more concern when it's uncertain that he can save him, such as when Madam Red almost killed Ciel or when the Undertaker threatened his life.  He is often critical of his master and how he treats his workers, but he has admitted that he does not dislike the butler lifestyle despite all the troubles he goes through on a daily basis.

Sebastian views money as "rubbish" and "materialistic." He is fluent in French and Latin, enough to teach the latter. He is also proficient in Deutsch (German) and East Franconian, a dialect in Southern Germany. Sebastian has a great and mutual distaste for Grim Reapers. Moreover, he profoundly adores cats and had kept at least thirteen hidden in his wardrobe to prevent Ciel from finding out.

Sebastian often contemplates the behaviors and attitudes of human beings. He regards humans, in general, to be "difficult creatures," but he is fascinated with their struggles and lofty goals. He is intrigued by how seemingly insatiable they are, for they are willing to "drag others down to get what they want, even in death."