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. The manga shows him to be more expressive, particularly when enraged and his facial features are exaggerated to demonstrate as such (possibly as a way to display his normally unreadable emotions). In the anime, his expressions change slightly and his tone of voice tends to be consistent at nearly all times, accentuating his monotonous demeanor.

His inflated ego and delusional sense of self worth are so large that he cannot stand being mocked or corrected in any fashion, believing that others only exist to serve him. It also means he rarely ever takes any opponent or threat seriously. He is very self-conscious of his unnaturally sickly appearance, immediately following with a lengthy denial that he is unhealthy, sick and much less on the verge of death, in all likelihood a bitter reminder that he is still imperfect.

While ill he is constantly self conscious. He yearns to leave his room, and get away from the confines of his household to be with you. He loved you more than anything, to the point he would kill someone if they hurt or flirted with you. Obsessive and controlling, he wants you by his side all the time.