I guess my when I predicted that this mysterious girl was going to be revealed as someone who "admired the her old friend" I supposed I underestimated what the author was going to do.
It looks like we have a full blown lesbian stalker here. The author even seems to label her as a "Crazy Psycho Lesbo". It's only a matter of time before she attacks the MMC claiming it's his fault that the woman she loves is now a neet and that he's awful and that all men are awful and the FMC is this perfect being that only she is worthy of.
I just hope she's only here for one arc. But I know manga authors (and certain readers) tend love these kinds of characters. For what logical reason, I don't know. Characters like this chunnin woman only serve to annoy the reader and impede the progress of the main relationship with the only thing they contribute to the story is yuri-fanservice and a running joke of "Look! She's gay and she's hates men! Isn't it funny that she wants to sleep with FMC and thinks the MMC is trash?"