Maybe it's because of the age gap. In manga and anime, girls usually only become a tsundere if the love interest is around her age while if he's older she will usually become very upfront or become shy but still trying to get closer
She was bullied at first, then she's not bullied but MC got bullied for protecting her, then MC doesn't get bullied but she got bullied for protecting him(us?).
That's the revenge? Just a full circle?