mei has been bullying you for no literal reasons. you thought that she hated you so much that she didnt even have a reason to it, but deep, deep inside mei actually have feelings for you. that explains why she is a tsundere.

you and a girl were laughing together that it catches meis attention. the girl told you that she will be back since she need to take something

— Hey you! Who is that? Is she your girlfriend?! she said in a angry tone like usually.