Before Hajin went to off Nayun's oppa, they have lots of bonding moments in their group outings with the other Main Cast and some other date like stuff. Hajin and Nayun were never alone, always with Suho, Yeonha, Jonghak, and the class president (that tall boy who was always with Suho). But that did not stop Nayun from catching very serious feelings for Hajin because Hajin helped her a lot. Hajin also helped Yeonha, Rachel, and Suho many times in between him helping Nayun. At one point after Hajin offed the oppa, they realy did had a "real date" because Nayun kept pulling Hajin where ever she goes as coping mechanism in her grief and depression. Hajin is drowning in guilt from what he did to her oppa at this point and just shoved it all away and had that date with Nayun hoping that they will never find the one who offed her oppa... but some masive interuption happened and it led to another sh** in "another arc". Nayun became the train wreck to end all train wrecks after these 2 events. After that "another arc", Nayun went dark berserker mode because her detective senses pointed to Hajin as her oppa's life subscription ender. That was also the time when Yeonha and Nayun became closer friends. That was also when Hajin dropped out from Cube and went with Boss.