Even though she's also being manipulated and might have some mental health issues, Yeonju does strike me as a very selfish person. It's not that the signs aren't there about her bf, with his behavior and how often it happens, she'd have to be in a coma to not sense something is wrong. However, she chooses not to see it because it doesn't fit the love story narrative that she has. She may not know the whole story but she knows Hari has been troubled and is uncomfortable, but she turns a blind eye for her own happiness. No matter what anyone says, no matter what happens, she'll justify, explain or avoid it because it doesn't suit her. At the end of the day, I don't even think it's about her being madly in love with her bf. In truth, I think it's more of an obsession with having things fit the image she has in her mind of her life/relationships.