Okay, so, crazy theory time now that the story is getting somewhat serious again.
1. Shaayu only started falling for Itsuka a month ago.
This one is only a theory because the manga hasn't explicitly confirmed it, but the start of chapter 2 (I think) gave us a huge hint in that direction, where it was stated that it was only a month ago (so a month before the events of chapter 1) that the Hero realized that she found Itsuka beautiful.
It's important because it means that for an undefined amount of time, she didn't and so Istuka was exposed to her flirtatious behaviour with others while never registering that way to her, so he catalogued her as a playgirl and her sudden feelings for him can only be pretend or even him reading too much into her.
2. Itsuka (feel)s an impostor
This one is probably self evident, but our MC suffers from a heavy dose of impostor syndrome. He depreciate himself and doesn't see himself as anything remarkable by himself, instead wanting to resort to external means of popularity (looks, getting kicked out of the party, etc.)
Combined with the insane amount of respect he has for Shaayu (he calls her The Hero in this chapter, which denotes a distancing between himself and some sort of idolized Hero) it prevents him from seeing her feelings for him.
Now the source of this insecurity could be interesting. I can see several reasons :
*His emotions always prevented him from using magic correctly where he learnt it and he was thus bullied for it until he managed to "contain" them.
*He lacks the mana for his feats and his earring is actually a stolen artifact that produces his mana for him, making him feel like a fraud.
*He comes from a family of physically strong peoples but was the runt of the litter and learning magic wasn't enough to escape the mockeries and all that.
3. Shaayu's feelings for Itsuka are unhealthy and are the cause of the rampages.
The last two chapters have hinted at it. Shaayu is getting frustrated that Itsuka doesn't get her feelings. And we saw that she doesn't want anyone from "noticing" Itsuka, she wants him for herself only. Meanwhile she ignores pretty easily Toifa's feelings.
And I think her suppressing her frustration and bad emotions is backfiring on everyone.
Shaayu is super strong, that much is clear and her abilities as a Hero aren't only physical (it seems she can produce wings of light as seen in chapter 2). But she rarely uses those and it's possible she doesn't really control them. And what caused Hero-chan to rampage was a luminous mark on her neck, not something dark. And it happened just after that Shaayu showed us the unhealthy side of her attachment to Itsuka.
Anyway, that's all for me folks !