Kimetsu no Yaiba theory *contains manga spoilers*
Ok so I have a theory about upper Moon demon 1. You know, this guy?
At first I thought he was the same person who was in Tanjiro's inherited memory but now I don't think that's the case. I think that they are two different people.
This man (the one who was in Tanjiro's memory, and also the one who came very close to killing Muzan):
Is a different person than the man who would eventually become Upper Moon 1. I believe this for several reasons.
There are two panels indicating that there were two swordsmen in the past that bore similar if not identical appearances. These panels are not insignificant and the marks appear on our pillars later.
2. From the above panel it seems likely that the two swordsmen with the fire mark could be either brothers or possibly even twins. One of them would go on to become Upper Moon demon 1, and the other would give the earrings to Tanjiro's ancestors and teach them the dance of the fire god. The fact that Upper Moon Demon 1 said he had blood relation to Muichiro, and Muichiro had a twin.
3. Upper Moon Demon 1 uses "breath of moon." The information we've been given about "breath of sun" would lead us to believe that the person in Tanjiro's memory would use sun, not moon, so that's another reason to think they are different people.
4. The swordsman in Tanjiro's memories had fallen into a depression about something. Being unable to stop a close relative from turning into a demon? (IE: upper moon 1)
5. The newest chapter also mentioned that people with the mark do not live past age 25. That may have been the motivation for upper moon 1 to become a demon. I still think they are different people however. It doesn't make sense for the same person who almost killed Muzan to turn around and join his side.
If anyone has anything else to add to this theory I'd be glad to hear it!
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