When adding “ah” behind someone’s name, it means that you’re calling them in an informal/casual way, it’s how one friend normally address another, to show that they’re close. Up until this point, Eun Sang only addressed Young Do by his full name, and at that point, Young Do was overwhelmed by emotions. Calling his name in such a dear way, “Young Do ah,” I feel like when Eun Sang called him that, she was trying to calm him down and talk to him as a friend, but Young Do who was so angry at that time, couldn’t handle it and told her not to call his name like that because it shook his heart. The pain in Young Do’s expression was just too much for me. awman.