I'd think she is just a weirdo cosplayer, but the fact that mc believed from the beginning that she's grim ripper makes me think that she legit is one, and the author doesn't want us to doubt it.
My dude, if someone holds manga hostage, then no one complains that someone takes it. People hate groups that hold manga hostage.
But what does it have to do with this manga? No one holds it hostage, last chapter was not even a week ago.