it’s the girl that boris couldnt save. she committed suicide after being raped so boris felt guilty because he could never help her or never did anything to help her. so in turn boris become vigilant about things like that and when he saw sehyun get raped in the restroom he tried to save him bc it reminded him of tracy
Wait but what does Sehyun have to do with her
he’s thanking her because if tracy was never in boris life boris would have never saved sehyun when he was getting raped. sehyun sees himself in tracy they are meant to be parallels in the story
Who’s Tracy