So, assuming there is infact a demon in Darkwick, as per my previous posts because I cannot get the idea out of my mind:
... I have a couple ideas.
Firstly, the demon probably isn't Leo, despite his nickname. I mean, it could be, he's mean as hell, likes spicy food, tried to kill MC, and I think Hyde threatened Sho with blackmail at some point? (Take that specific part with a massive cavern of salt because I haven't gotten there in my reread and I do not fully remember it!) Which could maybe possibly be taken as threatening to out Leo but I am very very iffy on that. It is a massive stretch even if I'm remembering the scene correctly. Point is, I don't think it's him bc him and Sho are "childhood friends" or smth and it said ONE person is a demon.
Second, it said one of you. It never really specified students. Granted, if it did it wouldn't narrow down much.
Thirdly, we haven't actually seen a demon in the story so far. (To our knowledge) (Not counting whoever might be a demon in the school.)
... So who are the suspects?
One suspect is Subaru. Hear me out here-
He's an anomaly sympathizer. Demons are, simply put, more or less a class of anomalies. When he was standing under that scissor tree, his eyes were more pinkish red than their natural... Purple. And we never got any follow up on that. Also pretty sure he killed someone??? The problem with this, is that he also has a past. He was famous, he did something, he grew up somewhere. For me, having a confirmable past kinda makes it seem less likely that any of these people are demons.
Second, Haku. Snake eyes, suspicious in general, no clue what his Stigma is and the game seems to be purposefully branding it as a secret we aren't meant to know about, which would make sense considering demons probably don't have Stigmas. A lot of his general suspicions seem to be easy to jot up to him being a medium though, so who knows. But like.. we can't really confirm or deny his family. If he's trying to distance himself from them, it could be him trying to avoid anyone fact-checking that he's really related to them, and being a medium would be a good cover for a demon since they're kinda opposites. Plus Taiga calls him an eel, so.
Thirdly, Tohma. Not incredibly much evidence for him, but we can all admit that he's suspicious as hell, I believe he said at one point he has a deal with Jin of some sort? The problem with this is that it's not a lot to go off of, and he has a stigma.
Next, we have Towa. His eyes are reminiscent of that of a ram, which demons are heavily associated with. His Stigma is iffy in what it can and can't do, same with the drawback. He could seemingly only talk at night, but when he's under the car it seems like any shadow counts. He's known Ed for a while, apparently, which is odd since Ed is a VAMPIRE, and nothing even seems to imply he ever had a life outside of Darkwick.
Fifth, Taiga??? The eating anomaly meat thing is weird but that's also kind of all there is to it.
Six, Dante. He came back from actual death. That's it, that's the theory. What the fuck is up with that.
And lastly, seven, Elias. Granted, he could also just be a spy for the Institute, I wouldn't doubt that, just.
Taiga seems very very suspicious of him, (institute spy or demon) he wanders around a lot as if looking for something? (Institute spy or demon...ish) His eyes are very weird (institute ROBOT or demon. Or maybe just a ghoul thing) and he is seemingly the only ghoul left from Dionysia that isn't Haru. His consequences were WAY lighter than those of the people who are LITERALLY MISSING. (Institute spy)
Or maybe I'm an idiot and it's the construction cat we saw once in the very beginning. Who knows