@IShallReadTheHeavens "fuckboi" is the term used by
@Ryomi, so I was just copying that.
Went back and re-read the first two chapters. Nobuto was Reika's first boyfriend and most likely the first person to pay romantic attention to her. Once they got in a relationship, she noticed he was belligerent when drunk, but didn't want to break up with her first love. Then he hit her and the day afterwards her dad told her that he said "I'm gonna kill that bitch Reika". Almost immediately after learning he said that, Reika decided to break up with him albeit while crying over losing her first love.
Did she mess up when judging the character of a handsome older guy flattering her? Yes. Did she stay in a verbally-abusive relationship for weeks? Also yes. But after he hit her and made a death threat, she decides to break up with him almost immediately (although she doesn't know he's already dead at that point). Considering most domestic abuse goes on for years, Reika did a pretty good job getting out as soon as she did (with the emotional support of her mom). Shame that she didn't realize earlier, but honestly she's the victim here. Besides, even if she got out sooner, the Yakuza would've still kept coming after her for her grandparents' inheritance.
All that being said, she's pretty unlikeable given her naivety in a mystery/thriller manga. And of course she's always gonna fall short compared to her incredible parents who've already been through this shit before with the mountains cult. It doesn't bother me too much though — makes perfect sense that a high school freshman would act this way.