@Memnon
I might be terribly wrong about this, but the way I see it, even if Junu's behind a mask with his stealth shenanigans, he is being genuine about his character, and does it for the best interests of Sung Ah. Sung Ah on the other hand, approached him only at the behest of of a random chat partner, befriended him not out of her heart, and then establishes a "not so important" gloomy acquaintance/friend view on him.
This connection proved to be that fragile after this chapter's revelations, acting on her perception of Junu after a tip from her "ranchat partner". She didn't really try to know whether Junu, the guy she chose to stand up for back then, is really that kind of malicious person. However, I will credit to Sung Ah on her trauma regarding stalkers, so that is a significant counterpoint to that. But for the little things, befriending someone when you really don't mean it; not trying to look deeper into the bullying, choosing to rather befriend the bully more closely because maybe (?) he's the more extraverted one. Things like that make her seem pretty shallow to my eyes.
While Junu kinda did, hiding behind a mask, he still did it with good intentions for her. The other one just judges the cover. This is just my take, and I kinda feel that I'm actually reaching out on things too much, so feel free to correct me if you feel that I'm wrong, I might learn a few things too.
*Edit 1: But yeah, I can't bring myself to blame Sung Ah 100% here either, but I still think she's shallow, unlike Hamin.