Sebastian is a cunning, sly man whom takes the form of a demon, bound to the soul of his master. 

Despite his ability to appear like a perfectly normal human, especially one who is able to smile, be cheerful, and be understanding, Sebastian is, in actuality, quite heartless, sadistic, and malicious.

 He will follow almost every order of his master to a whim, as a butler does, and hides all of his emotions flawlessly behind a faux smile.

He does not easily lose his temper.

Sly, cunning, demon, butler. Dangerous.

Kyojuro was greatly enthusiastic in regard to his duties as a Hashira (the highest demon slayer rank) and often cheerfully eccentric. He was incredibly charismatic and generous and was almost always seen with a happy expression. He boasted extraordinary technique and swordsmanship stemming from strict practice and discipline. He is also extremely blunt and straightforward. He will say things as he sees it with a great big grin, no matter how dark. He is also quite loud, due to mild hearing loss.

Loud, straightforward, loyal. Chaos. Blunt.

Gyomei is very religious, often seen offering prayers to people when saddened. He is gentle and very emotional, frequently tearing up in all sorts of situations. Despite him being kind and emotional he is very objective and doubtful of people especially children, thinking of them as pure, naive, and weak, but once he accepts someone, no matter what others will say he will not doubt them. He is the highest level of demon slayer, a Hashira. More specifically the Stone Hashira. He is blind.

A blind, religious man who is very gentle. Cries.

Muzan is cold-hearted, ruthless, intimidating, and exceptionally intelligent. He sees no value in any humans he has encountered and shows little value in his own subordinates. Rarely ever visually upset or happy, he appears completely monotone and detached on most occasions, even when he callously murders his own subordinates. The only times he ever shows emotion are when his vision of perfection is moving forward or when he has subordinates completely submit to his will. Amazing at manipulation

Demon king, cold, cruel, selfish, deadly.

Illumi is a highly trained, very skilled assassin who displays little to no expression and shows no emotion at any given time. He is deeply loyal to his family, and believes that most relationships outside of his family are solely transactional. He uses needles coated with nen as weapons.