Though they too will refer to someone as a 'baka' or 'pervert', tsundere characters will often pout, and may stutter or indirectly compliment their crush when someone mentions that character positively. One can tell the difference between a tsundere character and someone who simply hates another character by their expressions and dialogue. A tsundere character will cope with their romantic feelings towards someone by insulting them, denying having any interest in them, and making up excuses as to why they are interacting with them.
When it's above a certain threshold then they are considered tsundere. Every student has a value for how tsundere they are.
