Muzan Kibutsuji/Relationships

Tanjiro Kamado
Tanjiro is considered to be Muzan's main nemesis throughout the series. After murdering the Kamado family, He was personally sought out by Tanjiro in Asakusa, Tokyo when he recognized his scent. Although he had never met Tanjiro before, he knew immediately that he needed to eliminate him after recognizing his hanafuda earrings. In an attempt to flee, he created a diversion by transforming a young man into a demon to deter Tanjiro, but not before Tanjiro professed his goal to chase Muzan to the ends of the earth to kill him. Enraged by his threats, Muzan would send multiple demons to kill Tanjiro such as Yahaba, Susamaru, and Enmu to name a few.

It is later revealed that Muzan's desire to destroy Tanjiro is stemmed from his hatred towards his previous nemesis, Yoriichi Tsugikuni. After seeing the earrings Tanjiro wore in Asakusa, Muzan realized that he was a Breath of the Sun user, a technique developed by Yoriichi. Throughout history, he had made it his mission to eradicate all Breath of the Sun users in order to destroy the technique, since it had almost killed him at one point. Which is why, upon seeing Tanjiro he felt threatened and became frantic in trying to kill him.

Nezuko Kamado
Muzan tried to murder her and the entire Kamado Family after realizing their connection to the Breath of the Sun, but for some reason turned Nezuko into a demon through either a mistake or intentionally. He has since found her to be a nuisance much like Tamayo who was able to escape his control and attempted to kill her or Tanjiro. But, after Nezuko, developed a resistance toward sunlight a natural weakness of demons, Muzan didn't hesitate for a moment to attack the Demon Slayer Corps alongside the remaining Upper Moon Demons in an attempt to devour Nezuko and achieve his long-sought desire to become truly invincible.

Yoriichi Tsugikuni
When he first encountered Yoriichi, Muzan was arrogant and stated that he wasn't interested in Demon Slayers who use Breath Styles before attacking Yoriichi. However, much to his surprise, Yoriichi struck him multiple times after the former's blade turned crimson, and none of his wounds were able to heal. Muzan was forced to support his own detached head as well as forced to flee from Yoriichi. It was at that point that Muzan developed a loathsome fear against the Breath Style Progenitor, prompting him to convert Yoriichi's older twin brother into a demon and systematically murder other Breath of the Sun users.

Even 400 years later, Muzan's fear against Yoriichi remained as fully intact as simply seeing hanafuda earrings caused him to recall their clash. Although Yoriichi was long dead, his visage appeared in the blood memory of Daki after Tanjiro awakened his Demon Slayer Mark causing Muzan to be upsetted.

After the scars of the wounds inflicted by Yoriichi are revealed, Muzan believes that it is Yoriichi and not him the one who is a true monster.

Tamayo
In their initial encounter, Muzan turned Tamayo into a demon to cure her disease without telling her about none of the consequences of doing so causing the death of her husband and kids. Much like Nezuko, Muzan views her as a nuisance to him after being freed of his control and desires her to be killed. He didn't hesitate to mock Tamayo about killing her husband and children as well as going on a rampage out of despair killing other humans in the process placing all the blame on her without taking responsibility for any of it.

Twelve Demon Moons
Muzan holds these demons in high regard compared to the others, giving them a portion of his blood due to their deeps and to increase their powers. Although compared to the Upper Moon, he didn't treat the Lower Moon in the same manner not hesitating to kill all of the remaining ones after Rui, criticizing them for their weakness and then killing all of them expect the Lower Moon One. Although Muzan is more close toward the Upper Moon, he doesn't tolerate failure from either of them as Akaza left Tanjiro and the others alive despite his reasons being the rising of the sun, which would have killed him and even after killing Kyojuro Rengoku. However, it is later revealed that he viewed them as tools despite the loyalty all of them showed toward him.

Kokushibo
Despite him being the older brother of Yoriichi, the Demon Slayer who almost killed him, Muzan turned Michikatsu into a demon likely out of spite toward his twin brother.