Tanjiro's connection to Yoriichi is not through blood relation, but through their similar appearances, which is due to the divine nature of Sun Breathing.
Conclusion: Kokushibo, the upper moon rank 1 demon and Yoriichi are not Tanjiros' father and neither are Tanjiro's relatives. However, Kokushibo and Yoriichi are siblings and respect each other a lot despite Kokushibo hating the entirety of his brothers' being.
Why did Tanjiro never meet Upper Moon 1? It seemed like an obvious set up given he was Yoriichi's brother and Tanjiro is essentially Yoriichi's successor. So, I was shocked when the two never met or fought.
However, Tanjiro is not a descendant of Yoriichi Tsukiguni. Tanjiro is the descendant of someone who knew Yoriichi personally and was a good friend of his as well. Sumiyoshi was rescued by Yoriichi when a demon threatened his life. Since then Yoriichi often met him and they became close acquaintances.
There is no direct relationship between Kokushibou and Tanjiro. Namely, Tanjiro couldn't have met Kokushibou since Kokushibou is much older than Tanjiro, and there is no direct family relationship between them.
Demon Slayer's season 3's latest episode has left fans on edge with Tanjiro's discovery of Yoriichi Tsugikuni's Type Zero sword.
The episode begins with Tanjiro encountering the ghost of Yoriichi Tsugikuni, the creator of the Sun Breathing technique. Through a series of flashbacks, we learn about Yoriichi's tragic past and how he met the Kamado family.
Tanjiro Kamado is the only (known) living user of the Hinokami Kagura, but unlike the Water-Breathing he learned from former Hashira Sakonji Urokodaki, Tanjiro learned Sun-Breathing from his and Nezuko's late father, Tanjuro Kamado. So the question now is: who was Tanjuro Kamado?
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 ...
Tanjuro Kamado was the name of Tanjiro Kamado's father. He passed away before the event of the main story of the Demon Slayer.
As the young slayer aids in picking off more and more of the Twelve Kizuki, Muzan finds just as many reasons to be scared of Tanjiro Kamado. Tanjiro also ends up being the one to land the final blow on Muzan's original body, further proving why Muzan is scared of Tanjiro throughout the series.
Shinobu Kocho was the former insect Hashira who played a big role in defeating Doma, the Upper Moon 2, during the Infinity Castle arc. He also happens to be a descendant. Reason of death: Killed by Doma (Upper Moon 2).
Nezuko is at least stronger than Daki, meaning that if Muzan could bring her back under his control, she would certainly qualify for the most prestigious title a demon can hold: a member of the Twelve Kizuki. Thanks to Nezuko's remarkable strength, she would have to be an Upper-Moon.
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 One: Kokushibo
Muichiro, Genya, Sanemi Shinazugawa and Gyomei Himejima fought together against Kokushibo, though Muichiro and Genya were no match for him and were killed. Sanemi and Gyomei decapitated Kokushibo, but he successfully regenerated his head just as Akaza had.
After the deaths of Daki and Gyutaro, Kaigaku was promoted to become Upper Moon Six of the Twelve Demon Moons due to his power and confronted Zenitsu as a rematch he always desired.
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 aforementioned Hashiras are leagues above Tanjiro and they were easily overwhelmed by Kibutsuji Muzan's strength. One of Tanjiro's biggest strengths is that he can use the Breath of the Sun, but Tanjiro alone cannot kill Muzan given the Demon King's ridiculous rate of regeneration and raw strength.
In his own franchise, Tanjiro is outclassed by most of the nine Hashira as of the end of season 1, including Giyu Tomioka. It's easy to compare Tanjiro's strength to Giyu's — since they are both demon slayers — and they even use the same element: water.
Tanjiro certainly did a lot of good with his Water Breathing techniques, but his ability to use Sun Breathing both gives him an edge in battle and allows him to stand out among the incredibly powerful Hashira he fights alongside.
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.
During Tanjiro and the rest of the Hashiras' showdown with the Demon King Muzan, Tanjiro is mortally wounded and dies. At the same time, Muzan receives a particularly fatal lil' booboo himself. As Muzan is about to shuffle off the mortal coil, he uses the last of his powers to turn Tanjiro into a demon.
In the Demon Slayer manga's final chapter, 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.
Tanjiro's ancestor was a good friend of Yoriichi and arguably was one of his only true friends. Therefore Yoriichi gave the earrings as a gift, and since then they have been passed down the generations until they reached Tanjiro.
The mark on Tanjiro's head is actually a Demon Slayer Mark. Powerful Demon Slayers possess the ability to unlock unique marks that coincide with the special breathing fighting techniques they perform.