A tsun-tsun! Or, well, I guess it'd be tsundere instead, but!

[she looks way too excited to explain this for her own good.]

A tsun-tsun is someone who's always cold and kinda angry to everyone, which is the opposite of dere-dere and that means someone who is really lovey dovey. So a tsundere is someone who's kind of mean at first, but they're actually really nice and caring inside!