Kokushibo is one of the strongest demons in the anime, only second to Demon King Muzan. Before turning into a demon, he was a part of the Demon Slayer Corps alongside his brother Yoriichi Tsugikini.
The only character who was stronger than Muzan was Yoriichi. He almost beat Muzan before splitting his body into 1800 parts in an attempt to escape.
Muzan passes on his feelings and powers to Tanjiro, and he successfully turnsTanjiro into the new Demon King.
Many Kimetsu no Yaiba fans deduce that Yoriichi is Tanjiro's father, but that isn't the case. However, Yoriichi is responsible for teaching Tanjiro's ancestors the Hinokami Kagura, which has been passed down for generations.
In the official Demon Slayer Wiki, Tanjiro is considered the Strongest Demon while in Demon form, and he's got all the talent and abilities to compare him to the top Hashira, so basically he's legit the top slayer or would have been if they did not disband the Pillars/ Hashira positions.
Akaza was also one of Muzan's favorite demons out of all his subordinates. The reason behind this is Akaza's extreme loyalty to his master.
Yoriichi Tsugikuni was the first Demon Slayer and also the one who invented Breathing Techniques in the first place. Yoriichi was born with the Demon Slayer mark and prowess in swordsmanship. His powers were beyond comparison as he defeated Muzan once and had him on the verge of death until he managed to escape.
The two known demons who broke free of Muzan's curse are Tamayo and Nezuko. Tamayo was able to break free of the condemnation when Yoriichi fought Muzan, nearly killing him in the process. During this weakened state, Yoriichi let Tamayo go, free her from Muzan, and free her from the curse.
The warrior Muzan feared was a swordsman named Yoriichi Tsugikuni. Yoriichi was the only slayer before Tanjiro who came close to killing Muzan, and the flashback of their fight occurred in Chapter 187.
Muzan, the King of Demons, is the strongest and also the primal demon in the world of Demon Slayer. Consuming his blood is enough to turn you into a powerful demon. He can control all the demons in existence, with only people with strong willpower like Nezuko being an exception.
The demon slayers successfully defeated Muzan.
After various battles within the Dimensional Infinity Fortress the remaining Upper Moons are killed by the hands of various Demon Slayers. The last member, Upper Moon Four Nakime, ends up being killed by Muzan Kibutsuji himself to stop the Dimensional Infinity Fortress from being controlled by his enemies.
Some of the characters who have faced Yoriichi and survived
This happened due to a treatment that had gone wrong. While he was being treated, Muzan killed the doctor because he was not able to observe any results.
The only thing he's weak to is the sun. That is why he turned other humans into demons and spread his blood so that one of them might find a solution against the sunlight, so he can become truly perfect.
Muzan Kibutsuji has done some terrible things in Demon Slayer, but his slaughter of Tanjiro's family is one of the most mysterious.
Sanemi managed to survive the battle against Muzan, if just barely. Unconscious and straddling the edge of death, Sanemi chose not to go to Heaven with his siblings, but to lovingly join his mother in Hell because she'd killed most of her children after she became a demon.
Haruka Shiraishi: Muzan's Daughter.
Tsukito, Yorogi, Kuraku, and Isatsu all agree that they are loyal to Muzan.
Anne was originally planned to be Muzan`s older sister, who died by the hand of a Demon Slayer but that idea was scrapped, as the author knew practically nothing about Muzan`s early life. Instead, Anne became his wife, by which he had two children.
Tanjiro Is Strong, and Stronger than Zenitsu as he uses Sun Breathing.
Gyomei Himejima, the Stone Hashira, is the strongest Hashira in the Demon Slayer corps in terms of strength and swordsmanship. He is the only person who uses spiked flails and an axe instead of standard swords to kill demons. Such a weapon takes immense precision and soundness to use without inflicting personal damage.
Demon Slayer's season 3's latest episode has left fans on edge with Tanjiro's discovery of Yoriichi Tsugikuni's Type Zero sword.