I would mark this as containing spoilers but the trailer and imdb synopsis says this sort of info...
Story is about an ultra loser simp lord who falls in """""love"""" hint: his teenage penis says that a lady is attractive and he gave him some little attention. Now, since he is a loser without a job, friends, family, anyone, even the smallest gesture of friendliness makes him fall in love. This might sound crazy but it's actually EXTREMELY common among boys and men.
As for the girl/woman, she has mental problems, just like most women. Real problems... huge problems... she is a serial killer.
The guy falls in "love" with her... for her looks or per personality? Well he implies her personality... but what IS her personality? We only get 2 traits about her: she is friendly to him, she is pretty, a loner like him, she threatens to kill him and she is a serial killer.
It's pretty obvious. He is a loser who doesn't care about dying, in fact he tried to kill himself at the start of the movie. And he absolutely does NOT accept the fact that she is a serial killer... this narrows it down.
So do you think that a pretty lady who is a loner and is friendly to a guy would have a hard time finding a guy? What if you added the "serial killer" part? She would've wanted a guy who accepts her, not who wants to change her. He SAYS (lies) that he loves her for who she is but doesn't accept her main passion. He's just desperate, he knows he can't get another girl because he's a loser.
This situation, as shown in the movie, sounds absurd, but if you take the serial killer part, it's extremely common. The guy wouldn't "love" her, he's just desperate and would "put up" with her passion. The girl knows she'll never find someone who accepts her for who she is, so she "settles" for this guy who "puts up" with her.
This kind of relationship is doomed. Absolutely toxic and they're not truly happy with each other, but they're teenage loners who refuse to grow up and learn anything. In fact this is the main reason I give it 3/10. The plot is pretty original... but the characters act like such absolute cringy idiots. Then again, there wouldn't be a plot at all if they had a double digit IQ. It makes it look remarkably similar to some weeb fantasy, despite not being an anime.