@BlueCatMage I, too, don't fully understand but...
If I got it right it's more like those characters all had a fixed personality during the game period, and now that the game ended it seems like they got a human-like personality, not "yandere" or "tsundere", but yes people who think.
Our MC is acting, for a really really long and exhausting time that she's a perfect noble. Not to mention the environment she was born in — with parents that see her as a tool —, she knows the worse will happen to her (the game's ending always bad for her), all the while having the memories/ideals/dreams of an 21st century Japanese otome girl.
I think the mask represents her fake persona as noble that she's been holding ever since her reincarnation, always smiling and now she simply breaking