Honestly, I think that the author keep leaving hints about who or what is the mc is but they didn't give us a solid answer bec sometime it's more fun to not know everything.
Since we don't know what exactly is the mc (a human, a soul, a demon etc) it's make the story more interested. The author probably want us to keep guessing and make our own headcanons.
For me, I feel like not knowing what she is make her less op unlike how it show in the manga that she can just slaughter anyone since I don't know the exact source of her power, if it truly her power or it was something else that lend their power to her.
Ok I hate to ask this, but I'm a bit lost... What exactly is mc?? Is she a demon, a reincarnated person from modern times?? Or like does she have a split personality disorder?? I'm so fucking confused, it could be due to the translation errors here and there but I'm also a bit slow so that could be the main problem but like can someone explain to me what the fuck is mc