Nah, the fiancee is just a hikkomori with bad habits and the dad has a master plan to heal her and fix his son's attitude (let's be real, Eun-Joo IS kind of a douche). It was addressed why he wouldn't marry her either (he still loves his deceased wife and a relationship to another woman is the same as cheating on her). The dad is not an idiot, he's actually really clever about the whole thing. Eun-Joo just has a LOT of pride and Ma-Ri has some seriously bad habits, but at least she's honestly trying to fix that.
EDIT: Need to also add that despite the artwork, he's a middle-schooler and middle schoolers are stubborn brats. He's true to his values and education, but is easily swayed by others, like a kid. He's his own antagonist of the story, but I think it's just because he's not done growing up.