I think he is a demon in the literal sense, but not figuratively as the other two (❀╹◡╹)ﾉ
I think he's a demon and he's even said as much. I suspect the twist to be more on demons being purely evil and something to be eradicated. We're getting a lot of philosophical monologues on duality and balance. Light is bright but offers no respite from pain whereas shadow offers solitude and privacy. Night is when humans dream not the day.
I think the story is going for balance. That demons are natural and necessary for humans just as humans are necessary for demons. ML and MC were both doomed if things continued being dominated by the light. But, with the saint accepting the night and promoting balance they can both be saved.
i have a feeling that duke baine is not really a demon and like prob. the most safest among the three