The manga Kimetsu no Yaiba by Koyoharu Gotouge, accessible in English through Viz Media, includes significant Demon Slayer spoilers.
One of the most significant and enigmatic figures in the mythology of Demon Slayer: Kimetsu no Yaiba is Yoriichi Tsugikuni. Yoriichi is without a doubt the GOAT of Demon Slayers because he invented Sun-Breathing, the first and most potent of all Breathing Techniques.
Though he died many years before the events of the series, his Sun-Breathing is carried on by Tanjiro, who now possesses the most powerful Breathing Style and bears a striking resemblance to the Original Sun-Breather. Yoriichi, like many others in a demon-infested world, had a tragic life, but his relationship with the Kamado family provides an unexpected twist on one of Demon Slayer's most frequently asked questions: are Yoriichi and Tanjiro related?
In Demon Slayer, Who is Yoriichi Tsugikuni?
Yoriichi was born with the Mark of a Sun-Breather in a time before Breathing Styles even existed. He didn't speak until he was seven years old, leading his family to believe he was hard of hearing. His mother would pray to the Sun gods for him to be able to hear and gave him the Hanafuda Earrings as charms.
Due to his talent for seeing into the Transparent World, he was a skilled swordsman even as a little child. Yoriichi has the ability to peer inside the bodies of others and watch the movements of his adversaries' muscles. Yoriichi encountered members of the Demon Slayer Corps much later in his life, after his wife and unborn child were slaughtered by demons, and taught them Breathing Styles, but none of them could use Sun-Breathing—not even his twin brother.
When Yoriichi met Muzan Kibutsuji, he compared the Progenitor of Demons to a volcano's erupting magma, boiling and seething as if to consume everything. He realized right then and there that he was born with the power of Sun-Breathing solely to defeat Muzan. In fact, Yoriichi defeated Muzan in an instant with such a flawless display of swordsmanship that Kibutsuji's cells still tremble when reminded of the encounter hundreds of years later. Unfortunately, Yoriichi had a question for Kibutsuji before eliminating him, allowing him to flee.
Yoriichi saw himself as a failure for letting Muzan slip through his fingers at the same time his own brother decided to become a demon known as Kokushibo, Upper-Rank One of the Twelve Kizuki. Yoriichi saw Sun-Breathing as a divine power bestowed upon him, and it bothered him that he had failed to use his gift to rid the world of demons while his own brother died. Tanjiro Kamado, who is not related to Yoriichi, bears a striking resemblance to Yoriichi due to the divine nature of Sun-Breathing. Muichiro Tokito is the only living blood relative of Yoriichi; the Mist Hashira is the descendant of the child that Michikatsu Tsugikuni, Yoriichi's twin brother, left behind when he became a Demon Slayer and later a demon.
Yoriichi and Tanjiro: Are They Related?
The Kamado family was saved by Yoriichi while he was still a Demon Slayer, and he remained a close friend of the family. He paid the Kamados one last visit after failing to vanquish Muzan and seeing the transformation of his brother into a demon. Suyako (the mother) wanted to witness the Sun-Breathing forms while he was there, and Sumiyoshi (the father) committed them to memory while Yoriichi did them. The Kamado family made a promise to give Sun-Breathing to their offspring in recognition of their obligation to Yoriichi when he handed up his Hanafuda Earrings because they knew they would never see him again.
The fact that the Kamado men share Yoriichi's auburn hair color and that Tanjiro now sports the identical Hanafuda Earrings Yoriichi once wore seem to be nothing more than a coincidental coincidence. The forms of Sun-Breathing were transmitted by the Kamados with almost perfect accuracy, but it wasn't until Tanjiro that one of them received the Mark of a selected Sun-Breather.
Yoriichi and Tanjiro are linked by their selection to wield Sun-Breathing, but Yoriichi died long after his wife and child, leaving no direct descendants. Yoriichi and his family have been reincarnated and are seen in the deep background of a panel as Tanjiro's own descendants head to school in the final chapter of the Demon Slayer manga.