In my thoughts, I think Miya is jealous to hear about trialing prince all the time from Prez, thinking something along the lines of "Why does she always talk about him... I'm here too". So she always held these views on him.
Something-something happened in middle school, I think Sora just, as usual, selflessly helped Miya on her problems in the literature club, but of course Miya would reject him outright, like she hated his guts. Also she's distrustful of everyone else around here. So, to mitigate this, he joined literature club to become the worst member so the slander will go to him instead. Of course Miya would view this as incredibly dumb, because why. But at least she knew that Sora IS actually kind, really kind to the point of selflessness. With how Miya responds to Sora that hard, I think she's insecure of herself. She felt jealous of Sora being a trialing prince, but at the same time can't compete with his kindness, so she responds by 'being bratty'. I think with the added bonus that his selflessness is sometimes kinda dumb so she's being cynical about it.
We can see that Miya promptly asks Prez to buy drinks, and already know what to buy in the supermarket, knowing that Sora jumped head first again to helping someone. At least she cares enough that he's a good guy, to a fault.
What I thought maybe that Sora fell in love with how Miya wrote her stories, as the senpai said "Wish coming true" tropes and all. Because of this Sora jumped head first into helping her in a roundabout way, because he thinks her stories are not the worst. Maybe the stories reminded him of his brother, or his point of view to help others. Idk, i'm just coping out here. Maybe it's that type of "first love at first sight" on how Miya wrote her stories or smth.
I don't like tsunderes like her, but I don't hate her.
Rooting for Prez all the way!
