The warrior Muzan feared was a swordsman named Yoriichi Tsugikuni. Yoriichi was the only slayer before
Between all of Tanjiro's skills and his mannerisms, Muzan is certainly scared of Tanjiro. Tanjiro represents and certainly ends up being the core threat to Muzan's quest to step back into the sunlight.
Yoriichi was in a Selfless State when facing Muzan. This meant that he had no emotions, bloodlust or animosity, making it very difficult for Muzan to gauge his true powers or predict his moves. it's so satisfying to see muzan still so scared of yoriichi, even centuries later.
So the only reason Muzan is afraid of Tanjiro is because of the Hanafuda earrings he possesses, which acted as a reminder of his downfall. nxt. Hanafuda Earrings are a symbol of Sun Breathing users, traditionally passed down from generation to generation as a family heirloom.
It is revealed that Muzan's true fear of dying and immense desire to live and survive was due to him being born "dead," where his heart would constantly stop in his mother's womb and was assumed stillborn until he fought to breathe before his cremation.
Curse of Kibutsuji
This curse besotted any who partake of Muzan's blood must be careful. The curse prevents Muzan's name from being spoken aloud, and any who do will meet a horrible demise. One demon, Susamaru, faced this reality when battling Tamayo.
The only Demon he despises the most is the Upper Rank Two: Doma.
Who does Muzan regret killing? He shows his most human side to fans, wherein he admits to regretting murdering his doctor. He comments that had he been patient, the doctor would have cured him, and he would never have turned into a demon.
Yoriichi Tsugikuni, the first swordsmen to ever use Breathing Styles, once nearly killed Muzan with Sun-Breathing, and Muzan has been both physically and psychologically scarred ever since.
Anne was a kind and loving soul and was described as the only person that Muzan had truly loved, besides his three daughters.
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.
Rimuru Tempest becomes a God towards the end of the series and can travel back in time and travel between worlds. His space-time abilities alone would be enough to beat Yoriichi.
But if we are talking about the human form, then it's a big fat NO, because Tanjiro has just mastered the Sun Breathing whereas Yoriichi is the inventor of it, also he's way more powerful and holds an unfathomable battle intellect, larger than any Demon Slayer in history, so in conclusion, no one is remotely close to ...
Muzan is very cunning and intelligent. He is perceptive enough to be able to seamlessly blend into human society and he even started a family. He can hide his evil side and put on a friendly facade if necessary. He is manipulative and can quickly get people on his side for him to use them.
1 Yoriichi Tsugikuni (Demon Slayer)
Yoriichi's mastery of the Sun Breathing technique, combined with his incredible speed and agility, made him a formidable opponent for Muzan.
5 He Was Weak As A Human
Muzan conducts himself the way he does in part because he's trying to overcompensate for the time he was an extremely weak human being. As a human, he was always weak and unhealthy. His condition had deteriorated so much that he was almost on the verge of dying.
Yoriichi lived till old age, likely due to him having been born with the mark and thereby avoiding the curse altogether. Gyomei on the other hand did not activate his Mark until he was already older than 25. It was speculated that when he awoke his Mark during battle that he would likely die within hours.
In the first few arcs of the series, the identity of the demon that killed Tanjiro's family was a mystery. It is later revealed that it was none other than the series' main antagonist, Muzan Kibutsuji.
Thus, Muzan created demons for hundreds of years even when he didn't want to turn someone. Since he didn't know anything about the Blue Spider Lily medicine, he needed others to find the plant for him by happenstance. This is the general reason why Muzan turned Nezuko into a demon.
During Japan's Heian Period, roughly 1,000 years prior to the Taisho Period, during which the anime and manga are set, Muzan Kibutsuji was declared stillborn after his heart stopped beating multiple times inside his mother's womb.
1) Yoriichi Tsugikuni
His powers were beyond comparison as he defeated Muzan once and had him on the verge of death until he managed to escape. Even in his eighties, he was still in his prime as he slit the throat of his twin brother Michikatsu Tsugikuni, who is turned into a demon and goes by the name Kokushibo.
Kokushibo was the final Upper-Rank to die during Demon Slayer's "Infinity Castle" arc. Muichiro, Genya, Sanemi Shinazugawa and Gyomei Himejima fought together against Kokushibo, though Muichiro and Genya were no match for him and were killed.
Akaza is an upper-rank-three Demon and one of the most faithful minions of the Demon king Muzan Kibutsuji.
Doma is always smiling and acts very friendly, but this cheerful, caring nature is simply a facade. He states that he cares for his fellow Upper Moons and even cries when he learns that Akaza died, though, in fact, he doesn't feel anything for them. Even when he was human, Doma didn't feel any type of human emotion.
Hantengu like all members of the Twelve Kizuki is shown to be faithfully loyal to Muzan and following each and everyone of his orders and demands without question despite Muzan's usual demeaning and condescending attitude towards him.