WARNING: The following contains spoilers for Demon Slayer: Kimetsu no Yaiba Season 2, Episode 13, "Layered Memories," now streaming on Funimation, Hulu and Crunchyroll.
The battle between Tanjiro Kamado and Daki got existential in the sixth episode of Demon Slayer: Kimetsu no Yaiba's "Entertainment District" arc. Enraged to a dangerous point by Daki's violent massacre of an entire city block, Tanjiro's icily calm demeanor as he cut Daki to pieces stirred a memory in the Upper-Rank demon -- except it belonged to Muzan. Seeing a vision of a man who appeared to be a user of Sun-Breathing, Daki had the same question as the audience: who was that man?
Each and every demon that walks the Earth has some of Muzan Kibutsuji's genetic makeup in their body. As the Progenitor of Demons, it's only by his blood that a human being can be transformed into a demon. This has allowed Muzan to have omniscient knowledge of and omnipotent control over his demons, but the memory experienced by Daki is a new piece of information regarding the extent of her master's influence. This particular memory, however, is probably something Muzan would rather not have been seen by anyone, even one of his most trusted creations.
As Tanjiro coldly condemned Daki and her treatment of human beings, the Upper-Rank demon was struck by the memory of another man giving a very similar message, just not to her. Daki quickly identified the memory as Muzan's, in which a man with the same auburn hair, Hanafuda Earrings and forehead scar as Tanjiro delivered an equally foreboding speech to him. Given his numerous similarities to Tanjiro, it's clear the mysterious man was a fellow user of Sun-Breathing, the original and most powerful Breathing Style in Demon Slayer.
There was a brief flash of the man charging forward, sword in hand, implying that he actually fought Muzan. If Daki's sudden trembling -- which she confirmed came from the memory, not herself -- is any indication, this fight is an unpleasant memory for the Progenitor of Demons. Clearly this mystery Sun-Breather failed to kill Muzan, but that the latter's very cells reacted so strongly to the reminder gives the impression it may have been a close call. Given the raw strength of Tanjiro's powered-up Sun-Breathing against Daki, if the mystery Sun-Breather was a master, it stands to reason that he could have dealt some vicious wounds to Muzan.
As for this mystery Sun-Breather's identity, only context clues are given to form any sort of conclusion. According to Shinjuro Rengoku, a forehead scar like the one Tanjiro and the mystery man both wear is the mark of a Sun-Breather. Given that Muzan is 1,000 years old, and Sun-Breathing appears to have been all but lost outside of the Kamado family, this is likely quite an old memory. Considering Sun-Breathing is the original Breathing Style, and this man appears to have used it to harm Muzan ages ago, Daki may have been seeing the face of the first-ever user of Breathing Styles.
When Muzan first put a hit out on Tanjiro, he instructed his demons to look for "the boy with Hanafuda Earrings." This memory confirms the villain does indeed know the significance of these earrings, and explains why he wants Tanjiro dead but hasn't done it himself. Muzan Kibutsuji, the Original, Progenitor and King of Demons, might actually be afraid of a young coal-burner who just wants to turn his sister back into a human. Hopefully, Tanjiro survives his latest brush with death in one piece so he can continue to master the Hinokami Kagura -- because the very notion of Sun-Breathing's existence strikes fear in the heart of the most powerful demon in Demon Slayer.
