I think Rui was Muzan’s favorite.
If any of other lower demon moons had died I don’t think Muzan would have cared as much. I mainly think this because Muzan had Susumaru and Yahaba believing they were apart of The Twelve Demon Moons.
If any other Lower Moon would’ve died then Susumaru or Yahaba would’ve been a good replacement for them. Why else keep those two demons so close to him and at his beck and call?
Yes, he has others that are close to him like Nakime and before that he had Tamayo but they weren’t Demon Moons. They both knew they were demons just serving closely by Muzan’s side. Yet... he let Susumaru and Yahaba believe they were apart of The Twelve Demon Moons or at the very least Susumaru believed that.
Besides that we also know the Lower Moons get replaced quickly.
So having a few demons in line to fill that position isn’t that odd.
Lower Moon Six was a very new Demon Moon.
So new he hasn’t been inside the infinity fortress in fact.
Or it could just be the rendezvous wasn’t there before considering there was also the drum demon, Kyogai. Who I’m pretty sure got replaced by the new Lower Moon Six and Nakime.
Although Kyogai’s blood demon art was a weaker version of what Nakime could do and he was also a past Lower Moon so it could be a different reason Lower Moon Six didn’t recognize the location. Though I like to believe Kyogai was replaced in both aspects.
Kyogai also shows us that there’s more than one way to not be apart of the Lower Demon Moons (I’m saying Lower Demon Moons taking into account the Upper Moons have stayed the same for hundreds of years.).
Which is showing how useless you are to Muzan. I can’t think of why Muzan wouldn’t kill Kyogai right there besides the fact that stripping him of his title would be more embarrassing. This would have Kyogai be more motivated to serve Muzan.
With that in mind it makes more sense to just replace Rui. Muzan had done it before but also this is a reason that makes me think Rui might’ve been someone Muzan favored.
When he called all the Lower Moons he specifically said Rui’s name then addressed him as Lower Moon Five.
Not saying he didn’t know the others names but for someone like Muzan who is all about respect I think it’s very important. He does of course refer to the Upper Moons with their respective names but I think that just might be because he’s known them for decades and they’ve done well to serve him unlike the lower moons.
With Rui’s death also came the dismantling of the Lower Moons.
Rui’s death might’ve not been the reason for this but is sure as hell was the catalyst that lead to this. Muzan had thoughts of abolishing them before but didn’t do it until after Rui’s death. His death and the inconsistencies the Lower Moons had shown all piled to make his final decision.
When Muzan turned Rui I honestly believe he saw himself in Rui.
Rui was sickly just like Muzan. Muzan perhaps wanted to “save” Rui in the same way Muzan himself was saved from his sickness. Muzan was probably projecting his past onto Rui.
Even when Rui felt bad about murdering his own parents Muzan told him he shouldn’t feel that way and be proud of his strength.
Muzan may not care for any of the demons he creates besides the fact he can use them to his benefit but I do believe Rui was favored more than all the others.
