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 visibly 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. Initially, his expressions would only change slightly and only bear a slight shift from his usual calm attitude and his tone of voice tends to be consistent at nearly all times, accentuating his monotonous demeanor. His cruel and imposing nature has gained him the great admiration and fear of many demons. After Hantengu informs him of Nezuko Kamado's survival of sunlight, Muzan reveals a side that is in stark contrast to his perpetual composed disposition, as he is shown to be ecstatic at the prospect of absorbing her to finally gain the immunity to sunlight himself. In addition, he also shows himself to be extremely arrogant and prideful, first seen when he mockingly asked a platoon of soldiers sent to kill him if they were referring to him. His boastful side is also shown when he gloats to Kagaya about never facing any divine retribution for his millennia of murders. Furthermore, he is shown to have a sadistically cruel side to him, first shown when he reminds Tamayo on how she murdered and ate her family following her transformation, gleefully smiling as she wells up tears and further reminding her that she killed several people as well, rhetorically asking if he imagined that . In addition when Muzan has Nakime summon the Demon Slayers Corps inside the Infinity Castle, he sadistically boasts to Tanjiro that he will murder the entirety of its members that night, and when he responds with a vow to hunt him down, Muzan boldly dares the Demon Slayer to follow up on his threat if he can. It is heavily implied that after his encounter with Yoriichi Tsugikuni, Muzan's more vicious and overconfident personality was stifled by the sheer terror brought upon him by the Demon Slayer that nearly killed him, only resurfacing after learning of Nezuko's ability and realizing a new chance to complete his ultimate goal is now in his grasp. His inflated ego and delusional sense of self-worth are so large, he cannot stand being mocked or corrected in any fashion, believing that others only exist to serve him. This also means he rarely ever takes any opponent or threat seriously. He brutally killed a drunk man simply for being rude to him and killed the man's brother and a female companion when they had done nothing, all while proclaiming of his own perfection and superiority over them, displaying his shrewd pettiness as he won't settle for any mockery and will do anything he pleases until the humiliation is remedied. This event also hints that Muzan has unpleasant memories of his weaker, human past, and is very self-conscious of his unnaturally sickly appearance, immediately following his own rhetorical questioning 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. When he confronts Kagaya Ubuyashiki, the man responded to Muzan in a tone of pity and sadness for the life he had led. However, Muzan only interpreted his words as insults and threats, which further accentuates his hatred for humanity a

Zestial speaks in an archaic dialect redolent of early modern English. He has a very sophisticated way of speaking, and often keeps his temper rather than get easily frustrated, as the case when Velvette mocks him. However, despite his overall demeanor, he can be slightly petty, loudly sipping his tea to cut Velvette off mid-sentence before opposing her proposal to fight back against the Exorcists.

His reputation is such that demons other than his fellow Overlords are terrified of him, hiding or cowering whenever he passes by. The sight of him causes one civilian to set himself on fire out of fear.

Befitting his advanced age, Zestial is extremely patient and cautious, telling Velvette that they shouldn't take immediate action in spite of being given proof that Exorcists can be killed out of fear of the potential repercussions.

He is also shown to be compassionate to those with which he is close, sensing Carmilla's tension after an Overlord meeting and offering her an ear upon which to vent her concerns.

Illumi, for the most part, displays very little expression. In contrast to his expressionless demeanor, Illumi often speaks in a somewhat joyful and absent-minded tone. He is generally a stony, ruthless person with an extremely cold way of thinking and reflects solely on his own interests. Still, he holds a twisted and overprotective form of love for his younger brother, Killua; this manifests to the point that he used his Nen ability to control Killua in order to ensure his survival and obedience. He and his father raised Killua to believe that the only thing sustaining him was darkness and that he could only obtain joy in causing the death of others. As a means of protecting his little brother, Illumi implanted a needle in Killua's brain, forcing the latter to run from battles he wasn't sure he could win. Illumi cares for most of the Zoldyck Family members. He is willing to handle Alluka's power, even if it costs him his life. He has stated, however, that family members are not allowed to kill during an inner mission (a mission where there is an internal conflict), but insinuates that he would be fine with killing Alluka and does not consider Alluka a member of the family. While Illumi seems devoid of many human emotions, he is not without a sense of desire as he greatly hankers Alluka's wish-granting ability. Tsubone implies that Illumi's sinister ambition to control Alluka's power even caused him to stray from the path of an assassin. At some point, Illumi formed an unusual relationship with Hisoka, which he described as "give-and-take". He once came to the aid of the magician by standing for him as a double in the Phantom Troupe's hideout while Hisoka attempted to find Chrollo. However, when Hisoka asks Illumi if it would be okay if he killed Killua, Illumi displays a horrifying side of himself and asks if Hisoka wants to die then and there. His demeanor and bloodlust were very similar to how Hisoka reacted to Illumi's indecision on whether or not to kill Gon, whom he sees as a threat due to his closeness with Killua during the Hunter Exam, albeit on a much lesser extent. Despite the mutual benefit of their relationship, Illumi always believed one of them would wind up killing the other. He, therefore, accepted to form a contract with Hisoka with the condition that if the magician is killed, Illumi will receive his reward. The assassin cryptically refers to it as an "engagement ring" and "prenuptial agreement". Illumi casually commits mass murder every time he uses his abilities, sacrificing the lives of innocent people the vast majority of the time.He also joined the Phantom Troupe, an organization notorious for committing genocide,however, he doesn't appear loyal to them when he discloses that all the Spiders were on the Black Whale to Zodiac member,