I know that noonim refers to an older female in korean but like she's your girlfriend why would you call her that ? Ciel
Why wouldn't he call her that? It is a respectful way for males of addressing females older than ythemselves (which frankly I feel it's kinda rude to constantly keep track of who is older/younger but...) the issue is the translation, they picked the wrong or first option on the MTL
Why does he call her sister