Im lost

This!!
But im thinking MAYBE she was a female pasasite(?) Like he was already infected/carrying the thing but the process had to be started by him having sex with this female (?) parasite maybe? N then the thing needed semen to continue the cycle? Idk if it makes sense but he is the carrier?

I think I get it now. The first host is a small insect, probrably an ant. Then it moved to bigger hosts like the wasp and cat. This is a long process, that's why the Detective said there are an [unsually high number of dead insects in the room]. When this monster move to the next target, its old vessel split open in half and the monster will grow bigger while keeping a feature of its previous hosts. The cycle continues and that's why in the end it look like a big clump of different insect/animal parts.

About the girl, I think she's actually the real owner of this house, not the University boy. The story he tell the Dectective is a lie, it was the monster in him talking. I think the truth is, the monster eventually get to her cause she lives in the same house. Then it used her body and went to the party, take the University boy home, transmit itself to him via sexual contact. The rest is the begin of the manga where it hunt for the next host.

This is just my takes though lol.

Waittttt you might be cooking!! That would make more sense!!

It would mean it went from a woman to a small man to a larger man (our detective) rather than the weird way before

Thanks for sharing your theory!!

Adding on, looking again, the thing stands kinda like a person???

Damn thats a good theory, I always forget a character being an unreliable narrator is an option

That'd be horrifying! 0_0 And thank you so much for liking my theory heheヾ(☆▽☆)

Thank you for the kind words (๑•ㅂ•)و✧ This manga really got my brain working lelf and right to come up with that theory