Responses
I think Mari lashed out on Junyoung because that was something she didn't want to hear. It's true that Mari's boyfriend may not be taking her seriously, but Junyoung's not dating him to know how he really feels about her either... We could see that Mari is supportive of Sihoon and Junyoung being together and she's happy that he's "family" (lol). x))
All these people saying Mari is sweet and a good friend...Did you forget she literally lashed out at the MC just because he had the "audacity" of telling the truth: her oppa (the real boyfriend) is an asshole who cheats on his wife and likely doesn't care that much about her either.
I don't even now why he still hangs out with her.