Something that the "ugh, why didn't Rumi just tell Mira and Zoey about her patterns?" crowd likes to ignore is that, even setting aside the fact that the girls gleefully slaughter demons and say anything with patterns has to die (especially Mira on the latter), when Rumi does try to express her concerns with Takedown, she's shut down by BOTH of them, aggressively so with Mira. Yes, Rumi is beating around the bush about her true problems with it, but it's not until an ENTIRE TRAINFUL OF PEOPLE DIE PARTIALLY BECAUSE THEY WERE FIGHTING that they actually listen to her, because they weren't even really giving her much of a chance to explain herself beforehand! Why would Rumi risk telling them about her patterns during the movie given they aren't even listening to her about her problems with a song?
And I was watching a YouTube video the other day that said that Mira and Zoey mock Celine, and Rumi should've taken the opportunity to tell Mira and Zoey about her patterns at that moment....completely ignoring the fact that RIGHT AFTER Mira and Zoey mock Celine, they double down and are like, "yeah, but she's right, we have to hide this problem with your voice until we can fix it." You can even see Rumi get a sad, resigned sort of disappointed look in that scene right after they say that.
So yeah, if you can't tell, I really hate it when people bash Rumi for not telling Mira and Zoey her secret, or at least, when people insist that Rumi had ample opportunities to share her secret during the movie and had no reason to believe that Mira and Zoey wouldn't accept and forgive her.
It's bad situation overall. No matter what, if a significant time period passed between Mira and Zoey meeting Rumi and them discovering her patterns, they would be hurt and feel betrayed by her keeping such a massive secret from them regardless of the circumstances they discovered the secret, and Rumi (and Celine) likely think, not wrongly, that them discovering her secret, even if she just tells them, could cause a rift in their relationship and cause the Honmoon to weaken/the girls to be uncoordinated in being a united front against demons, putting Korea at risk.
But on the other hand, I can completely understand if Celine didn't want to start of her and Rumi's new relationship when they first met Mira and Zoey by saying, "Hello, you have been recruited to fight demons. Also, this girl that I've raised who will be part of your team is part demon." Setting aside the fact that teaching Mira and Zoey that there's complexity among demons could easily get them killed if they hesitated for just a moment while killing demon, I feel like telling Mira and Zoey that they've been recruited to kill demons with a half-demon would lead to them starting out their new mentorship under Celine by not trusting her or Rumi.
Either way, it's a shitty situation from all ends no matter how you slice it. But I'll firmly stand by the fact that it's completely understandable why Rumi gravitated towards Jinu, why she didn't tell Mira and Zoey about her patterns, and how their behavior reinforced her idea that they would not be happy and would not accept her if she revealed her secret.
I'm 100% open to the idea that Rumi is selfish in regards to her relationship with Jinu in the sense that she does not keep trying to kill him even though she SEES the staggering amount of people his plan is getting killed, killing him would be a devastating blow to the Saja Boys, and he straight-up tells her he is not a good guy and isn't the dude to help her, insisting that he's not redeemable (when I first watched the movie, I kept thinking, "Is she ever going to bring up the fact that tons of people are dying because of him?") but in regards to her hiding her patterns? I don't see her actions there as stupid and selfish, but rather intense self-preservation.