To me it will depend on how old she actually is... idt around 14 for example can be put into the she's a child that doesn't know better excuse. And also... Noona is also used by younger cousins and younger male friends in general... I get she feeling uncomfortable... but doesn't justify her actions to me... unles shes like 10 and just looks older...
I'm not sure... it's more a if she was around 14... I'd overlook if she was 10-11... but she doesn't look so... so idk her age...
Tbh bitc, ahole, prick etc would do too... as I answered in some other comment I'd get the outrage if bitc was being used to insult her in regards of promiscuity or diminishing her as female with the original context of the word... but that's not the case and it's just about the behavior... that would still be considered bitchy even if her are or gender was different...
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This is just my OPINION!!
I was just thinking that YES, this Maru girl is being super rude and harsh...
BUT maybe the word "noona" is triggering her because:
1. Our Gongja is still a stranger to her, so she's uncomfortable?
2. MAYBE her dead younger brother als called her "noona", so now it's a trigger word for the trauma of losing her brother OR it makes Maru think Gongja's trying to replace her dead brother...
I know I shouldn't be justifying her behaviour (since I think she's old enough to know right and wrong) BUT she IS still a CHILD. AND she hasn't seen her mum in a long time, so maybe she feels alienated around the new people in a changed home? ╮( ̄▽ ̄)╭