Chapter 1... Wow...
... Alright, I kind of expected a ecchi manga...
Not a full on sex one...
*looks at genre*
Romance? Being dumped is probably romance to some circles that I'm not a part of. Being prey for a curious self-proclaimed sis-con could be romance in some circles too, I guess? WTH?
*looks at content*
Ecchi? That seems about right. An understatement even. If I had to I'd add Smut, since 19, 20, 21 and 22 are pretty much him spying on intercourse.
*looks at theme*
Supernatural? Huh? Where? The fact that the dorm is being overtaken by vines and flocks of crows? Olden, uncared for, European style = Supernatural now?
School life? College. Seems to check out.
My speculations based on the tag and chapter 1:
MC got himself in some deep shit with succubusses that drain men of their energy and use it to boost themselves up for higher concentration, ability and beauty.
On that point, either Saki's sister spared the MC or she just shunned him because he wouldn't be of any use to her beside being a wallet for the "while" it lasted, probably a test period to see how well connected he is; to see if he's usable for anything.
this dude its an actual simp, fuck me, never thought i would use that word but damn, he straight up sees her run to another dude after getting dumped and he still tries to msg her and is worried about the tickets when its too late, damn, that little sister is sus too