Tanjuro Kamado a recurring flashback hero from
Despite not being a key character in Demon Slayer, Tanjuro had an important part in Tanjiro's life. Tanjuro was proven to be a remarkably powerful person, despite not being a demon slayer and having a sad sickness that renders his body terribly feeble.
It turns out that Muzan attacked the Kamado family and specifically turned Nezuko into a demon in an attempt to create a demon that would be able to withstand being in the sunlight.
Yoriichi had lost his wife and unborn child to demons, and he had a fateful encounter with Muzan Kibutsuji, the Progenitor of Demons. Yoriichi defeated Muzan in an instant, but he had a question for him before eliminating him, which allowed Muzan to escape.
He wasn't a Demon Slayer but Tanjuro was exceptionally strong. While on the brink of death with that horrible disease (which made his body incredibly weak and frail) he was still strong and fast enough to slay a bear more than two meters tall.
Moreover, Tanjiro's appearance somewhat resembled Tsugikuni. 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.
Upper Rank 1: Kokushibo
Kokushibo, the Upper Moon One, is renowned for his mastery of the sword and formidable array of abilities. Formerly a Demon Slayer, he honed his skills and created his own unique breath style known as Moon Breathing, which ranks as the second strongest breath style.
During one of his visits, Yoriichi performed all of his Sun Breathing techniques in succession in front of Sumiyoshi and his family. Sumiyoshi was so captivated by the movements that he decided to pass it down to his descendants as a way to keep Yoriichi's legacy alive.
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. More specifically though, was the fact that Nezuko was of Kamado blood.
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 refused to die and when he was finally about to breathe his last, he transferred all the blood to Tanjiro in the hope that he would become the successor and carry out his will.
Unfortunately, Gyutaro spots Inosuke and stabs him right through where his heart should be with a sickle imbued with deadly poison.
The demon slayers successfully defeated Muzan.
Tanjiro is a prime example as he only became a Demon Slayer and got his black Nichirin after a demon Human Slayer-ed most of his family.
As it turned out, the scar on Tanjiro's forehead did have some significance to it, rather than simply being a Todoroki-esque burn from scalding water. Tanjiro wasn't born with this scar on his forehead. Rather, he acquired it when tackling his younger brother out of the way of a falling pot of boiling tea.
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.
It is theorized that Nezuko was able to conquer the Sun because she comes from a lineage of Sun Breathing users and because she kept a big part of her own humanity i.e. because she never ate human flesh.
In episode 6 of Demon Slayer season 2, Nezuko entered a berserk state to protect her brother Tanjiro, causing her body to develop into that of an adult. She loses her gag, grows a single horn and gains vine-like patterns across her body.
Muzan's dream is to one day stand under the sun proudly and live his life freely. As for defeating the Demon Slayer Corps, he simply wants to annihilate any obstacle that stands in his way. Muzan keeps creating demons, hoping one of them might conquer the sun.
In the Demon Slayer Corps, the Hashira is a group of the strongest and most reputable swordsmen who have defeated members of the Twelve Kizuki. Yet, for some reason, Tanjiro Kamado, the series' protagonist, never becomes a Hashira.
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.
Muzan Kibutsuji (鬼舞辻無慘, Kibutsuji Muzan) is the main antagonist of the Demon Slayer: Kimetsu no Yaiba fantasy action manga and anime series. Muzan is the powerful King of all the Demons, who has the ability to inject his victims with blood that allows him to convert them into more Demons.
While she might not be as powerful as Muzan himself, her overall strength makes her easily comparable to an Upper rank demon.
Tanjiro decapitates Akaza with the Hinokami Kagura: Setting Sun Transformation. Akaza was bewildered, wondering how Tanjiro did not have any fighting spirit, comparing this feeling to fighting against a plant. He acknowledges that Tanjiro surpassed his speed.