Okay so here's my theory: As you might know, in Korean 'Nabi' means butterfly, on the other hand, butterflies symbolize a representation of resurrection, change, renewal, hope, endurance, and courage to embrace the transformation to make life better (source: worldbirds) meaning that Yoo Nabi will make Park Jae Eon re-think and change his own perspective of relationships in order to quit his unhealthy habit which is playing with people's feelings. Hopefully, it made sense!
oh bong yi is the most useless character in this drama. what's her purpose? i don't undertand...and she's so stupid...she…
could've been said in a nicer way but i respectfully understand where you're coming from. it's not that she's stupid but she judges way too quickly. though, entering a psychopath serial killer's house might put her at higher risk, wouldn't you desperately want to murder someone whom you trusted for taking a very close innocent family member's life?