I'm glad Mitsuno seems to be taking this supernatural business more seriously, and panicking less overall. It helps that he's so strongly oriented toward helping others in need, and that impulse is overpowering his fear and anxiety, seemingly even in "mundane" situations like the hotel room. (yeah he still panicked, but he pushed through it to resolve the incident, instead of just caving in a puddle of nerves like his two classmates from last chapter would accuse him of doing.)
Guessing this particular "monster" relates to repressed desire, though it seeming to specifically be about lust/sexual repression is interesting and not fully explained (I think). Probably a case of being told "you'll inherit this family name/legacy", and the specific subtext of her being a girl, and thus only doing so via an heir, while being constrained and railroaded and allowed no freedom--presumably even in her choice of life partner.
(A lot of assumption, but that's a song & dance played out before in 'well-bred families' in manga elsewhere, so it echoing the theme in this story doesn't feel like too much of a stretch, given what's been shown.)
Hopefully Mitsuno can save Nanami-san, though, since she's not fully-gone yet.
Thanks for the TL. This story's worth being patient for thus far.